\documentclass[preview,border=1pt]{standalone}
\usepackage{amsmath,amssymb,amsthm}


\begin{document}

\begin{align}
f(x) &= 4x - 3x^3 \label{eq:fdefinition} \\
g(x) &= e^x - \sin x + 2\cos x + x - 2x^2 \label{eq:gdefinition}
\end{align}
\end{document}