There is a mistake for your qns 10 answer. You should not multiply 7 on the left side to make it 63 as left side is a multiplication relationship and not addition/subtraction. Keep the 7 on the left side as denominator.

9/7 (3^x) x (7^x) = 21^2x
9/7 = 21^2x / 21^x
21^x = 9/7