(a) It states that energy can neither be created nor be destroyed.
(b) Given m = 1500 kg,
u = 36 km h^-1 = 10 m s^-1,
v = 72 km h^-1 = 20 m s^-1,
W = ?
Work done = Change in kinetic energy =1/2 m (v^2-u^2)=1/2 x 1500 x (20^2 -10^2) = 225000 J
The work done is positive.
(c) It has maximum potential energy at its extreme position and maximum kinetic energy at the mean position.