Let's look at the times when it is at 4m
-4.9t^2 + 18t - .4 = 4
-4.9t^2 + 18t - .4 - 4 = 0
-4.9t^2 + 18t - 4.4 = 0 multiply through by -10
49t^2 - 180t + 44 = 0
Using the Quadratic Formula
180 ± sqrt [ 180^2 - 4(49) (44) ] 180 ± sqrt [ 23776]
t = ____________________________ = _________________
2 (49) 98
The time when it is above 4m is given by
sqrt [ 23776] / 98 - - sqrt [ 23776] / 98 =
2sqrt [ 23776 ] / 98 =
sqrt [ 23776] / 49 sec