1) вынесем 3^x за скобку
3^x(9 - 3 +1) = 21
3^x * 7 = 21
3^x = 3
x = 1
2)Поделим на 9^x
9 * (16/9)^x - 7 * (12/9)^x -16 = 0
9 * (4/3)^(2x) - 7 * (4/3)^x - 16 = 0
t = (4/3)^x, t > 0
9t^2 - 7t -16 = 0
t = (7 +- √(49 + 576)) / 18 = (7 +- 25) / 18
t = 32 / 18 = 16 / 9
(4/3)^x = 16/9
x = 2