



 
Hi Abi, Put a coordinate system on your diagram with the Xaxis along the ping pong table and the Yaxis at the left of your diagram. Assume that the player serving the ball releases it $b$ centimeters above the table and at the same instant strikes it with the paddle moving in the positive X direction giving it an initial velocity in the X direction as $v_0$ centimeters per second. If you ignore air friction then the beginning path of the ball is part of a parabola of the form $y = ax^2 + b.$ Since the parabola opens downward you know that $a$ is negative. Treat the X and Y coordinates of the ball as independent functions of time t given by $x(t)$ and $y(t).$ In the y direction you have the equation of motion for an object under constant acceleration. From this equation find the time when the ball first hits the table. In the X direction you have an object moving at a constant velocity $v_{0}.$ What is its Xcoordinate at the time when the ball first strikes the table? This gives you the X and Y coordinates of the point where the ball first strikes the table. This point is on the parabola and hence you can determine $a.$ I hope this helps,  


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