Use the MVT for integrals formula: Avg value of f on [a,b] = ∫ab f(x)dx / (b-a)
(Btw, what we are doing here is finding the height of a rectangle of width (b-a) that has the same area as the area under the curve, f(x). This height is f(x)'s average y-value on that closed interval):
1/4⋅x4]-21 ÷ 3 = - 1.25

RAFAH A.
01/21/21