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


\begin{document}

\begin{align*}
f(x) &= 4x - 3x^3 & h(x) &= \cos x \\
g(x) &= e^x - \sin x + 2\cos x + x - 2x^2 & i(x)&= \sin x
\end{align*}
\end{document}