Align an equation in the enumerate environment

Question

This is what I would like to be able to do:

I would like to be able to align my equal signs while keeping the equation aligned with the (a).

But to align the equation with the (a), I saw that I had to use \displaystyle which is not compatible with the align environment which gives me this:

\documentclass[fleqn, 12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{1ex}
\usepackage[nodisplayskipstretch]{setspace} 
\setstretch{1.25}
\usepackage{siunitx}

\begin{document}

\begin{enumerate} 
    \item [(a)] $\displaystyle
            R_{eq}=R_1+R_2=300+100 \\
            =\SI{400}{\ohm}
            $
\end{enumerate}
\end{document}

So I tried to create an align environment but my equation is not aligned with the (a):

\documentclass[fleqn, 12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{1ex}
\usepackage[nodisplayskipstretch]{setspace} 
\setstretch{1.25}
\usepackage{siunitx}

\begin{document}

\begin{enumerate} 
    \item [(a)]
        \begin{align*}
            R_{eq}=R_1+R_2&=300+100 \\
            &=\SI{400}{\ohm}
        \end{align*}
\end{enumerate}
\end{document}

Thanks in advance for your answers.

Answer 1

I suggest you replace the align* environment with an aligned[t] environment.

\documentclass{article}
\usepackage{amsmath,enumitem,siunitx}
\begin{document}

\begin{enumerate} 
    \item [(a)]
        $\begin{aligned}[t]
            R_{\mathrm{eq}}=R_1+R_2&=300+100 \\
                                   &=\SI{400}{\ohm}
        \end{aligned}$
\end{enumerate}
\end{document}