To answer also to the comment by @MLT, there is an alternative to the standard cases environment, not too sophisticated really, with both lines numbered. This code:
\documentclass{article}
\usepackage{amsmath}
\usepackage{cases}
\begin{document}
\begin{numcases}{f(x)=}
1, & if $x<0$\\
0, & otherwise
\end{numcases}
\end{document}
produces
Notice that here, math must be delimited by \(...\) or $...$, at least on the right of & in each line (reference).