In an incline projection the horizontal motion is uniform motion (vx)= constant why?

it is constant because there is not any force that oppose the horizontal motion

but the vertical velocity is not constant because there is a gravitational force that oppose the motion .

here we know that ball will drop down by total distance 3.2 m

so here we know that

now using kinematics

now in the same time it will cover horizontal distance of 0.75 m

so speed of the ball will be