The Question says:

A model rocket is shot into the air and its ath is approximated by the relation h= -5t + 5t + 30 where h is the height above the ground in metres and t is the time elapsed in seconds.

a) How tall is the building?

This is what I did:

h=-5t+5t+30

h=-5(t^2-t-6)

h=-5(t-2)(t+3)

sub t=0

h= -5(t-2)(t+3)

h= -5(0-2)(0+3)

h= -5(-2)(3)

h= -5(-6)

h=30

B) WHEN WILL THE BALL HIT THE GROUND?

I thought I could sub h=0 into the equation for this so I did it as follows:

h= -5t(t-2)(t+3)

0= -5t(t-2)(t+3)

0= -5(t^2+t-6)

0= -5t^2 -5t + 30

I don't know where to go from here or how to finish the problem or if I did anything wrong. Please help.