Finding an average rate of change over a finite interval is a 7th grade slope question: (y2 - y1) / (x2 - x1).
f(-2) = - 40 + 10 = - 30 and f(2) = 40 - 10 = 30
So the average rate of change of f on [ - 2 , 2 ] = (30 - (- 30)) / (2 - (- 2)) = 15
To find at which x-values the instantaneous rate of change = 15, we need to differentiate using power rule:
f'(x) = 15x2 - 5 = 15
15x2 = 20 ; x2 = 4/3 and x = ± 2 / √3 or ± 2√3 / 3 in SRF