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.