Recall that for a given f(x), f'(x) is the equation for the slope of the tangent line. We are given two points on a tangent line, so we can use the formula for slope to find the f' here.
m = (y_2-y_1)/(x_2-x_1) = (-32-4)/(5-8) = (-36)/(-3) = 12 = f'(8)
Thus, f'(8) = 12
Please note that f'(x) measures the slope of the tangent line; it does not provide the entire equation for the tangent line. To find that we would have to use f'(8) in point-slope form. That is beyond the scope of this problem.