All you need to know about negative exponents is that they raise the number or variable to the exponent power, then take the reciprocal of that new expression. For example, if you have 5^-1, that would be the same as 1/5.
Now to the problem. (5f)^-3
Apply the -3 exponent to both pieces.
(5^-3)(f^-3)
(1/125)(1/f^3)
1/125f^3