Multiple line formula with two alignment points

You can use the aligned environment:

\documentclass{article} \usepackage{amsmath} \begin{document} $\begin{aligned} &\text{If } & a+x&= a\\ &\text{then } & (-a)+(a+x)&= (-a)+a=0;\\ &\text{hence } & ((-a)+a)+x&=0;\\ &\text{hence } & 0+x&=0;\\ &\text{hence } & x&= 0. \end{aligned}$ \end{document} 

You can use aligned or alignat, depending on how you want to control the spacing:

\documentclass{article} \usepackage{amsmath} \begin{document} \begin{alignat*}{2} & \mathrm{If} & a + x &= a, \\ & \mathrm{then} & (-a)+(a+x)&= (-a)+a=0;\\ & \mathrm{hence}\quad &((-a)+a)+x &= 0; \\ & \mathrm{hence} & 0 + x &= 0; \\ & \mathrm{hence} & x &= 0. \end{alignat*} \begin{equation*} \begin{aligned} &\mathrm{If} & a+x&= a\\ &\mathrm{then} & (-a)+(a+x)&= (-a)+a=0;\\ &\mathrm{hence} & ((-a)+a)+x&=0;\\ &\mathrm{hence} & 0+x&=0;\\ &\mathrm{hence} & x&= 0. \end{aligned} \end{equation*} \end{document} 

I know the current answers already answer this question. But here it goes another option. This using an array environment.

\begin{equation*} \begin{array}{lrcl} \text{If} & a+x & = & a,\\ \text{then} & (-a)+(a+x) & = & (-a)+a=0;\\ \text{hence} & ((-a)+a)+x & = & 0;\\ \text{hence} & 0+x & = & 0;\\ \text{hence} & x & = & 0. \end{array} \end{equation*} 

Make sure to replace \text with \rm in case that you do not want the text to match your current font style and want Roman for some reason. Hope it helps!

Edit: As suggested below, one should note that the lines are spaced more closely than in other environments. To control this (if you want to) you can add the desired line space after each linebreak, e.g. \\[1 ex]. My impression is that the array environment gives you more freedom, but this also comes with having to develop the skills to tailor it to suit your needs.