I typed in one order but they appeared in reverse order. Please help

Question

There are two parts to this code. First is the figures and calculations after. (Indicated by the two comments). But when I run them they appeared in the reverse order.

\subsection*{Figures}
\begin{center}       
%should appear first
    \begin{figure}[ht]
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/Ptime.jpg}
        \caption{Pressure vs Step}
        \vspace{4ex}
        \end{subfigure}
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/temptime.jpg}
        \caption{Temperature vs Step}
        \vspace{4ex}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/KEtime.jpg}
        \caption{Kinetic Energy vs Step}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/PEtime.jpg}
        \caption{Potential Energy vs Step}
        \end{subfigure}
    \caption{}
    \end{figure}
    \end{center}
    %should appear next
    \textit{Calculation of Self Diffusion Coefficient}
    \begin{equation*}
    \begin{split}
    \text{slope} & = 0.0021 \\
    \text{Self Diffusion Coeff} & =\frac{1}{6}\times \text{(Slope of MSD at long time)}\\
    & = \frac{1}{6} \times \text{0.0021}\\
    & = \SI{3.5e-4} {\text{LJ units}}
    \end{split}
    \end{equation*}
    \text{Considering the LJ liquid to be liquid Ar, we have $\sigma = 0.34 \textit{ nm}$, $\frac{\epsilon}{k_B}=120 \textit{ K}$. }\\
    \text{$\therefore \frac{{(L^*)}^2}{t^*}=\frac{{L^2}/{\sigma^2}}{t\sqrt{\frac{\epsilon}{m\sigma^2}}}\implies \frac{L^2}{t}= (\sigma^3\sqrt{m/\epsilon}) \frac{(L^*)^2}{t^*} \implies \frac{L^2}{\epsilon}=\num{2.49e-31}\frac{(L^*)^2}{t^*}$}
    \linebreak
    \begin{equation*}
    \begin{split}
    \text{Diffusion Coefficient in SI Units} &= \num{3.5e-4}\times \SI{2.489e-31}{\meter^2 /\second}\\
    &=\SI{8.7e-34}{\meter^2 /\second}
    \end{split}
    \end{equation*}

Answer 1

Works correctly once the text above the second eqn is shortened to just one text word considering --probably some error in the eqn --other than that lines up correctly

\documentclass[]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}   
\usepackage{mathtools}
\usepackage{amssymb,booktabs}
\usepackage{caption,subcaption}

\begin{document}
\subsection*{Figures}
\begin{center}       
%should appear first
    \begin{figure}[ht]
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Pressure vs Step}
        \vspace{4ex}
        \end{subfigure}
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Temperature vs Step}
        \vspace{4ex}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Kinetic Energy vs Step}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Potential Energy vs Step}
        \end{subfigure}
    \caption{}
    \end{figure}
    \end{center}
    %should appear next
    \textit{Calculation of Self Diffusion Coefficient}
    \begin{equation}
\begin{split}
TCU & = \frac{\text{purchasing cost} + \text{setup cost} + \text{holding cost}}{time} \\
    & = \frac{cy+K+h\frac{y}{2}t_0}{t_0}
\end{split}
\end{equation}
    \text{Considering the LJ liquid to be liquid Ar, we have $\sigma = 0.34 \textit{ nm}$, $\frac{\epsilon}{k_B}=120 \textit{ K}$. }\\
    \text{$\therefore \frac{{(L^*)}^2}{t^*}=\frac{{L^2}/{\sigma^2}}{t\sqrt{\frac{\epsilon}{m\sigma^2}}}\implies \frac{L^2}{t}= (\sigma^3\sqrt{m/\epsilon}) \frac{(L^*)^2}{t^*} \implies \frac{L^2}{\epsilon}=\num{2.49e-31}\frac{(L^*)^2}{t^*}$}
    \linebreak
    \begin{equation}
\begin{split}
TCU & = \frac{\text{purchasing cost} + \text{setup cost} + \text{holding cost}}{time} \\
    & = \frac{cy+K+h\frac{y}{2}t_0}{t_0}
\end{split}
\end{equation}
    \end{document}