\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align*}
    3+x &=4 && \text{we are trying to solve for } x\\
    x &=4-3 && \text{Subtract 3 from both sides}\\
    x &=1   && x \text{ must be one}
\end{align*}
\end{document}