We know by Euler's Formula that $\displaystyle e^{ix}=\cos(x)+i\sin(x)$. So $\displaystyle \begin{aligned}\sqrt[4]{7\left(\cos\left(\frac{\pi}{2}\right)+i\sin\left( \frac{\pi}{2}\right)\right)}&=\sqrt[4]{7}e^{\frac{\pi{i}}{8}}\\
&=\sqrt[4]{7}\left(\cos\left(\frac{\pi}{8}\right)+i\sin\left (\frac{\pi}{8}\right)\right)
\end{aligned}$
Do similarly for the second.