Acceleration (a) of a projectile is the same as the force of gravity, 9.8m.s-2. A rule to remember is that when an object is slowing down, the acceleration is in the opposite direction to the velocity. In this scenario the acceleration due to gravity would have been negative as it would be in the downward direction as velocity was 50 m.s-1 upward. Therefore accleration due to gravity is -9.8 m.s-2.
So 50 x (-9.8x2) = 30.4m.s-1