Do you mean: 7*3^x or (7*3)^x? For the first one, 7*3^x is not equal to 21^x because 7* (3^x) is not the same as 21^x. Now, this may sound confusing, so let's give x a value. If x = 2, then the expression would be 7* (3^2), which is 7*9, and that's not the same as 21^2 (42).
