if you mean: (2x^-3) then:
simple rules:
x^2 = x.x
x^-2 = (1/x)^2
a minus sign before your exponent just means you need to write 1/ (by whatever you need to calculate)
ex:
x^-3 = (1/x)^3
25^-2 = (1/25)²
4^(-1/2) = (1/4)^(1/2) = square root out of 1/4
...
if you mean (-2x)^3 then its simply (-1)^3 . (2x)^3 = ...