Hello I haven’t seen this formula S=ut+0.5at^2 before , what does it mean? :) thank you so much !!

Aside from the formula, i think you can look at the graph. The area underneath the V-t graph gives you the distance. As mentioned, the positive (upwards) gradient from t=1 to t=4 shows the ball increasing speed aka falling down to the ground before changing direction. This gives the height from where the ball is released from. Hence, by area under graph, distance=0.5x3 (from t=4 minus t=1 aka base of triangle) x 3 (max speed aka height of triangle)=4.5m