How will I figure out about the right formula to use? Please thoroughly explain the step by step process. Thank you

10) A bullet with a mass of 10.0 g is fired from a rifle with a barrel that is 66 cm long. a) Assuming that the force exerted by the expanding gas to be a constant 5603 N, what speed would the bullet reach? b) Do an energy bar chart analysis of the situation, with the entire gun and bullet as the system (and the gas created by the explosion outside the system). 12