The charge is losing energy as it moves from a higher potential to a lower potential. Work must be done on the charge to move it against the electric field. The change in energy can be calculated by multiplying the charge by the change in potential.
(a) The charge is losing energy because it is moving from a position where the potential is +2.0 volts to another where the potential is -15 volts. In other words, it is moving from a higher potential to a lower potential.
(b) Work must be done on the charge to move it against the electric field from the higher potential to the lower potential. This work is done by an external force or source.
(c) The change in energy of the charge can be calculated by multiplying the charge (4.5 nanocoulombs) by the change in potential (from +2.0 volts to -15 volts). The change in potential is -17 volts, so the change in energy is -76.5 nanocoulombs-volts.