Problem 16 should indicate that the average NET force is 28 N.  Rather than saying the average force exerted by the engine.

Jacob, both work. Average force allows for the force to change, but when the average is taken, and multiplied by the time, the impulse-momentum equation works to find the final velocity - and if the velocity was given, it finds the mass. Net force is also a valid way to say it.  John

John,

Maybe I am misunderstanding the description of the problem...  The problem indicates that the rocket is shot straight up and that the engine exerts an average force of 28 N on the rocket.  I take this description to mean that the weight of the rocket has not been netted out yet and would have to be subtracted to get the net force responsible for the change in momentum.

Jacob

Jacob, I see what you're saying, but we didn't give the mass of the rocket or the engine, so we can assume that the engine's weight is negligible compared to the mass of the other parts of the rocket. John