The governing equation is 

h(t) = -16t^2 + 32t + 48,

where t is the time after throwing, h(t) is the object height (elevation over the ground surface in feet, 
32 is the initial velocity in ft/s, 48 is the initial elevation, 16 =  a is the half of the value 
of the free fall acceleration in ft/s^2.

We need to find the value of t for which h(t) = 0,  or

-16t^2 + 32t + 48 = 0.

Divide both sides by (-16). You will get

t^2 - 2t - 3 = 0.

Factor the left side

(t-3)*(t+1) = 0.

The positive root t = 3 is the time we are seeking for.
The other root is negative, so we ignore it.

Answer.  It will take 3 seconds for the onject to reach the ground.