Answer
(a) $W = -30.4~J$
(b) $W = 30.4~J$
Work Step by Step
Note that when the book moves from a height of 0.75 m to a height of 2.3 m, the increase in height is 1.55 meters.
(a) Since the direction of the force of gravity is opposite from the displacement vector, gravity does negative work on the book. The magnitude of the work is equal to the change in potential energy.
$W = -mgh$
$W = -(2.0~kg)(9.80~m/s^2)(1.55~m)$
$W = -30.4~J$
(b) Since the direction of the hand's force is in the same direction as the displacement vector, the hand does positive work on the book. The magnitude of the work is equal to the change in potential energy.
$W = mgh$
$W = (2.0~kg)(9.80~m/s^2)(1.55~m)$
$W = 30.4~J$
