Cosmological density fluctuation dependence on scale factor

Question

I want to show that in the radiation-dominated era, $\delta_H^2 \propto a^{1-n}$. I know that the wavenumber corresponding to the Hubble scale goes like $a^{-1}$ and that $\delta_H^2 \propto k^3 P(k)$ and $P(k) \propto k^n$, but I am unsure of how to put these together to get the desired result.