here tennis ball is accelerated from initial speed zero to final speed 50 m/s
In this accelerated motion the distance moved by the ball is 0.5 m
So here we can use kinematics to find the acceleration of the ball during this distance

here given that
vf = 50 m/s
vi = 0
d = 0.5 m


now if we have acceleration during its motion so we can find the applied force by using Newton's II law



now the applied force will be 150 N on the ball by racket