A stone is dropped from a height of 64 feet. In how many seconds does it hit the ground? What is the speed at impact?

I don't think you guys can see the formula, but I want to know where did the -32 come from? If its suppose to be cause of -(1/2)64 then why is there still a (1/2) there and why does that (1/2) turn it into a -16?

My just don't understand cause the formula I was told is.

Y(T) = -(1/2)GT^2 + VoT + Yo.