Given
Inital velocity U = 50km/ hour = 250/18 m/sec
Final velocity =?
Acceleration = 4 m/sec^2
Time = 3 sec
Third equation of motion : (Final velocity)^2 - (initial velocity)^ 2 = 2 * acceleration * displacement
That is :
V^2 - U ^2 = 2aS
First equation of motion : S = ut + 1/2at^2
Calculating S :
Bringing units to same scale , we get : 50km/hour = 50*( 5/18) m/sec = 250 /18 m/sec
S = (250/18) * 3 + 1/2 ( 4 ) (3)^2
=(250 / 6 ) + 18 = 358/ 6
Putting S in third equation of motion, we get
V^2 - U ^2 = 2aS
V^2 - (250/18)^2 = 2 * 4 * 358/6
V is approximately 25.9 m/sec ~ 26
V = 26 * 18/5 = 93 km/hour