Step-by-step explanation:
side = 9 ft
the volume of a cube is
V = side³
if V decreases by a to V', then each side decreases by side - cubic root of V'.
V = 9³ = 729 ft³
V' = (729 - 0.3) ft³ = 728.7 ft³
side' = cubic root (728.7) = 8.998765263... ft
difference side - side' = 0.001234737... ft
so, the change rate at 9 ft is
0.001234737... ft/min.
