Find the derivative of f at x. that is, find f '(x). f(x) = 7x + 8; x = 5

We know that f(x) = 7x + 8. So, take the derivative of this function with respect to x.

This gives
f'(x) = 7

Now, you probably realized that the x value disappeared. This means that no matter what value of x you use, the derivative of this function is always going to be 7, even at x = 5. That is, the rate of change of this function is always going to be a rate of 7.