Question 311209
To find the time when it will hit the ground, set your function to 0 and solve for t.

-5(t^2)+ 20t + 25 = 0

Can you solve for t?

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I read your question again.  They gave you the position equation, and increased the gravitational constant to make the math easier.

For time, solve the position equation y = -5(t^2)+20t+25 where y = 0 (ground level). You'll get two solutions. Discard the negative one.

For the maximum hight, you want to know when the ball just starts coming down, ie, when it's vertical velocity is zero. The velocity equation is the first derivative in the position equation. Solve that where dy/dt = 0 to get the time at maximum height, and plug that time back into the position equation.