One mole of ammonia gas is contained in a vessel which is capable of changing its volume (a compartment sealed by a piston, for example). The total energy $U$ (in Joules) of the ammonia is a function of the volume $V$ (in cubic meters) of the container, and the temperature $T$ (in degrees Kelvin) of the gas. The differential $dU$ is given by $dU = 840 dV + 27.32 dT$.

(a) How does the energy change if the volume is held constant and the temperature is decreased slightly?

(b) How does the energy change if the temperature is held constant and the volume is increased slightly?

(c) Find the approximate change in energy if the gas is compressed by 150 cubic centimeters and heated by 3 degrees Kelvin.