i don't remeber us doing this in class at all, so i read the chapter relating to this subject many times, and it's confusing for me.
ok i know that a = 9.8m/s^2, initial velocity = 0, initial height = 2r
so...
x(t) = 2r + 0 + 1/2(-9.8)t^2
2r = -4.9t^2
sorry but i have no clue on...