the formula h= 15t - 5t^2 gives the height 'h' metres of a ball, 't' seconds after it is thrown up into the air.

(i) find the times when the height is 10m

i figured out how to do this part of the question, t = 1 or 2 by using the quadratic equation but i still dont understand the second part of the question

(ii) after how long does the ball hit the ground?

thanks in advance!