I believe that you meant:
h(t)=-16t^2+18t+18 (because -6t^2 would mean the acceleration due to gravity would be -12ft/s^2, which certainly is not the case on Earth :P )
Anyway, the ball will hit the ground when h(t)=0 so:
-16t^2+18t+18=0
16t^2-18t-18=0
16t^2-18t=18
t^2-9t/8=9/8
t^2-9t/8+81/256=9/8+81/256
(t-9/16)^2=369/256
t-9/16=±√(369/256)
t=9/16±√(369/256)
t=(9±√369)/16, since t>0
t=(9±3√41)/16 seconds
t≈1.763 seconds (to nearest thousandth of a second)
