Need help with these questions

If you answer right ill give brainliest, : )

If cos(2x) = tan^2(y), show that cos(2y) = tan^2(x).

Diane’s bank is offering 5% interest, compounded monthly. if diane invests $10,500 and wants $20,000 when she withdrawals, how long should she keep her money in for? round to the nearest tenth of a year.