A ball rolls off at a speed of 3.0 m/s and lands 0.40 seconds later. How far from the base of the desk does the ball land?

If we ignore air resistance, then the horizontal speed

of the ball doesn't change after it rolls off the edge. So

(distance) = (speed) x (time)

= (3 m/s) x (0.4 sec) = 1.2 meters.