The first thing you should do for this case is to write the kinematic equations of the object in the vertical direction.
For this case, we have
a = -9.8 m / s ^ 2
Vf = a * t + Vo
clearing time:
t = (Vf-Vo) / (a)
t = ((8) - (20)) / ((- 9.8)) = 1.22 s
the position of the object for that time and for when V = 8.0 m / s is
y (t) = a * t ^ 2/2 + Vot + I
y (1.22) = (- 9.8) * (1.22) ^ 2/2 + (20) (1.22) + 0 = 17.11m
answer
the projectile is 17.11m high when its speed is 8.0 m / s
