during free fall for 3.5 s:
v = g t = 9.8*3.5 = 34.3 m/s when we pt on the brakes
h = (1/2) g t^2 = 4.9 (3.5)^2 = 60 meters so far, now we stop
we are going at 34.3 m/s and slow down at 2 g
0 = Vi - 2 g t
so
t = 34.3 /19.6 = 1.75 seconds braking
h = Vi t - (1/2)(2*9.8) t^2
= 34.3 (1.75) - 9.8 (1.75)^2
= 60 - 30 = 30
(of course it will take half the distance we did in fee fall since we are braking at 2 g)
60 + 30 = 90 meters high
( I would make it a little higher if I were you :)