Hi Alexis.
When finding the value of a function with a given number, always replace the variable (for x) and simplify. As we can see in your problem, f(x) is being replaced with "4", so substitute "x" with 4.
This leaves us with:
5(4) - 5. All we need to do now is simplify.
5(4) - 5
20 - 5
15.
From the work above, we can see that the function of f(4) has a value of 15.
I hope this helps!