Here is the pdf of some absolutely continuous random variable:

\[ f(x)=\begin{cases} ax^2e^{-bx^2} & x\geq 0\\ 0 & x< 0. \end{cases} \]

Solve for \(a\) in terms of \(b\). So your final answer will look like

\[ a=\text{``some formula including $b$ and a bunch of constants''} \]