no full solution to homeworks, only guidance!

Replace the Laplacian s with jW where W means the omega. That is = 2 * Pi * f. Your Wc is  2 * Pi * f1.

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 = 1/squareroot(2) which means -3dB gain. Solving from that equation the frequency gives the wanted result.