Question 38731
Let us substitute the values as given and then solve for t...
h = vt - 16t^2
12 = 28t - 16t^2
16t^2 - 28t + 12 = 0
4t^2 - 7t + 3 = 0
(4t - 1)(t - 3) = 0
Thus
t = 1/4 sec and t = 3 sec