no full solution to homeworks, only guidance!
Replace the Laplacian s with j\$\omega\$ . That is =\$ 2 \pi f\$. Your \$\omega_c\$is \$ 2 \pi f_1\$.
You must derive a formula for the absolute value of the complex expression of the cascaded filter transfer function. You must set that to be =\$\frac{1}{\sqrt2}\$ which means -3dB gain. Solving from that equation the frequency gives the wanted result.