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well, under acceleration, the height h is
h(t) = (1/2)90.3 t^2
At t = 1.75, the height is 138.27 m
Since v = at, at t=1.75, the upward velocity is 158.03 m/s
Now it goes ballistic (really!) and the height is given by
h(t) = 138.27 + 158.03t - 4.9t^2
That's just a parabola. Find its vertex using your favorite method, and that is the maximum height.