Assuming the question is about this expression:
[tex] (4^{-5})^3 [/tex]
Use the exponent property
[tex] (a^b)^c = a^{bc} [/tex]
to rewrite the expression as
[tex] (4^{-5})^3 = 4^{-5\cdot 3} = 4^{-15} [/tex]
Now, recall that negative exponent behave as positive ones, except you have to invert the number beforehand (flip numerator and denominator). So:
[tex] 4^{-15} = \left(\frac{1}{4}\right)^{15} = \frac{1}{4^{15}} [/tex]
