the underside of a concerte bridge forms a parabolic arch that is 32m wide at the ater level and is 12m high at the center. the under surface of the bridge is horizontal and is 48m wide. the minimum thickness of the concerte is 1.5m
..there are three parts to the question
first you have to create an equation in the vetex form i got y=-3/64(x-24)^2+12
and then they ask
what is the vertical thickness of the concrete 4 m horizontally from the center of the bridge??
PLEASE HELP I HAVE BEEN STUCK ON THIS QUESTION FOR AN HOUR!!

Question

the underside of a concerte bridge forms a parabolic arch that is 32m wide at the ater level and is 12m high at the center. the under surface of the bridge is horizontal and is 48m wide. the minimum thickness of the concerte is 1.5m
..there are three parts to the question 
first you have to create an equation in the vetex form i got y=-3/64(x-24)^2+12
and then they ask 
what is the vertical thickness of the concrete 4 m horizontally from the center of the bridge?? 
PLEASE HELP I HAVE BEEN STUCK ON THIS QUESTION FOR AN HOUR!!