Aligned mathematics

\documentclass{article}


\usepackage{amsmath}

\title{Aligned mathematics}
\author{VK}
\date{\today}

\begin{document}

\maketitle

\begin{align}
	(x+h)^2-x^2 & = x^2+2xh+h^2-x^2 &&\text{(by distributivity)}\nonumber\\
    			& = 2xh+h^2 &&\text{(by subtraction)}\\
                & = h(2x+h)
\end{align}

\end{document}