The answer would be -0.012345679.
The first thing you would do is substitute 3 in for x, seeing that you've told us x = 3. After you've substituted x, the equation would look something like:
f(3) = -3^-4. From here, all you have to do is solve! -3 to the power of -4 equals -0.012345679. Therefor, f(3) = -3^-4 equals -0.012345679