Put one part of sentence outside $ math environment

I want the 'assume' written in normal style. So not within the math environment so it becomes italic.

This is my code

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\subsection*{1.2}
\paragraph{a)}
$
\begin{aligned}[t]
&h(x)=x^4\cdot(2x^2-1)^3 \\
&assume u=2x^2-1 \\
&h(x)=x^4\cdot u^3 \\
&h'(x)=4x^3\cdot u^3+3u^3\cdot u'\cdot x^4
\end{aligned}
$

\end{document}