to solve for the time it reach the maximum height, you must solve the first derivative of the function and equate it to zero
h(t) = −4.9t^2 + 14.7t + 1
dh/ dt = -9.8t + 14.7
then equate to zero
-9.8t + 14.7 = 0
solve for t
t = 1.5 s
then the maximum height is when t = 1.5
h(t) = −4.9t^2 + 14.7t + 1
h(1.5) = −4.9(1.5)^2 + 14.7(1.5) + 1
h(1.5) = 12.025 m