1/2 km = 500m, 3/4 km = 750 m.
From question,
500 = 50.ua + 1/2. a. 2500
or 500 = 50.ua + 1250a ...................eq(1)
ub = ua + 50a
Also,
750 = 50ub+ 1/2.a.2500
750 = 50ua + 2500a + 1250a ........................eq(2)
(1) and (2) are the two equations in two unknowns ua and a. Solve them to get the ans