Answer
(a) There is no change in the gravitational potential energy.
(b) Justin has done a total of $3400~J$ of work on the desk.
(c) The work that Justin has done has been converted into heat energy by the force of friction which has done $-3400~J$ of work on the desk.
Work Step by Step
(a) Since the height of the desk's position does not change, there is no change in the gravitational potential energy.
(b) We can find the work that Justin does on the desk in each direction:
$Work = F~d = (340~N)(5.0~m) = 1700~J$
Justin has done a total of $3400~J$ of work on the desk.
(c) All the work that Justin has done on the desk is equal in magnitude to the work that friction has done on the desk. As the desk slides across the floor, the force of friction does negative work on the desk. The work that Justin has done has been converted into heat energy by the force of friction which has done $-3400~J$ of work on the desk.
