A$1,200$kg$$rocket has a net propulsion force of $70$kN$($kiloNewtons$).$ Over a short time period, it uniformly speeds up from an initial velocity of$50$ m$/$s$$to a final velocity of$80$ m$/$s$.$ Assume that the mass of the rocket is constant during this time period and that the net force is along the direction of motion.

What is the$$change in speed$$of the rocket in m$/$s and in mph$?$