With the assumption that the acceleration due to gravity is 9.8m/s^2 and that the upwards direction is positive:
We can use the kinematics equation, [tex]v_f^{2}- v_0^{2}=2aS[/tex], where v is velocity, a is acceleration, and S is displacement. Initial velocity is given as 5.4 m/s. We know final velocity is 0, because we want to know the displacement at the peak of the tennis ball's motion. Acceleration is -9.8m/s^2. Just plug in these values to find the displacement:
