Rita hits a tennis ball with an initial vertical velocity of 15 m/s. when she hits the ball, it is 1 m above the ground. how long will it take for the ball to hit the ground?

We can use the  equations of motion under constant acceleration. Here the acceleration is due to gravity and equal to  9.81 m s^-2.

s = ut + 0.5gt^2

s = displacement = -1 , u =  15 m/s and g = -9.81 ms-2

-1 = 15t +0.5*-9.81*t^2

-4.9t^2 + 15t + 1 = 0

solving for t we get t = 3.13 seconds