If the speed is downwards, it is negative.
Acceleration is gravity and is also negative.
You should use the uniformly accelerated linear movement formula:
y=(1/2)at^2+vot+y0
Where y0=initial height, y=final height, t=time
If final height=the ground
then final height is 0
0=(1/2)(-10m/s^2)t^2+(-8.0m/s)t+30.0m
0=(-5m/s^2)t^2+(-8.0m/s)t+30.0m
Now you can use Bhaskara formula:
a=-5
b=-8
c=30
t=(-b+-square root(b^2-4ac))/(2a)
t=(-(-8)+-square root((-8)^2-4(-5)30))/(2*(-5))
t=-3.38 s-> IMPOSSIBLE
t=1.78 s->OK
