Given that the cannon ball is launched at an 18 degree angle
Initial speed = 25 m/s
So the initial velocity in the horizontal direction (say v_x) will be

The horizontal component of velocity will be constant throughout the motion since there is no acceleration in the x-direction, only acceleration due to gravity in y-direction.
So at the peak of its arc the velocity will be the same to its initial value, i.e.

Result: Horizontal velocity at peak of its arc will be 23.78 m/s.