The work energy Mark spent to move the car is
(force) x (distance)
= (140 Newtons) x (140 meters) = 19,600 Joules .
The car moves with constant speed. That tells us that its
acceleration is zero, so the NET force on the car is also zero.
The friction force must be equal to Mark's pushing force, in
the opposite direction, and friction does as much work on the
car as Mark does . . . 19,600 Joules. The work done by the
friction force shows up as heat.
