Giải thích các bước giải:

16.$\left(\left(-\dfrac23\right)^2\right)^x=\left(\left(\dfrac23\right)^2\right)^x =\left(\dfrac23\right)^{2x}$

17.$\left(\left(-\dfrac12\right)^x\right)^3=\left(-\dfrac12\right)^{3x}$

18.$\left(\left(-\dfrac12\right)^3\right)^x=\left(-\dfrac12\right)^{3x}$

19.$\left(2^3\right)^x=2^{3x}$

20.$\left(3^2\right)^x=3^{2x}$

21.$\left(7^2\right)^x=7^{2x}$