\documentclass{article}
\usepackage{mathtools}
\begin{document}

$\xrightarrow[\text{world}]{\text{hello}}$
$\xRightarrow[\text{world}]{\text{hello}}$

\bigskip    
$\xrightarrow[g(x)]{f(x)}$
$\xRightarrow[g(x)]{f(x)}$  
\end{document}