# Align an equation in the enumerate environment

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}


I suggest you replace the align* environment with an aligned[t] environment.
\documentclass{article}
\begin{aligned}[t] R_{\mathrm{eq}}=R_1+R_2&=300+100 \\ &=\SI{400}{\ohm} \end{aligned}