(g°f)(5) is equal to 16.
In order to find a composite function's value like this one, you first need to evaluate the first function. Since f is on the right side, we put the value in parenthesis in for x in that equation first.
f(x) = 2x - 8
f(5) = 2(5) - 8
f(5) = 10 - 8
f(5) = 2
Now once you have an answer in the first part of a composite function, we stick the answer we got in for the second part. Since g is on the left side, we put our answer (2) into g.
g(x) = x^4
g(2) = 2^4
g(2) = 16
Therefore, (g°f)(5) = 16
