A projectile motion is the motion of an object in two dimensions under the influence of gravity.
In this case, the object has no acceleration along horizontal direction, it has acceleration in vertical direction which is equal to the acceleration due to gravity of earth.
When the projectile reaches at the maximum height it travels only along the horizontal and thus it has only horizontal velocity at that instant.
Thus, the velocity of teh projectile at maximum height is same as horizontal component of velocity that meas 50 m/s.
