HOW HIGH DOES THE COLUMN HAVE TO BE, TO REACH FROM THE FLOOR TO THE DOME

Please solve question c).
The equation I am using is y = a (x - s)(x - t).

The dome over a town hall has a parabolic shape. The dome measures 48 m across and rises 12 m at it's centre.
a) Determine the quadratic equation that models the shape of the dome? y = -1/48(x-0)(x048)(Answered)
b) A vertical column needs to be attached to the dome at a point that is 4 m away from its rim. How tall is the dome at this point? y = 3.67(Answered)

c) THE DOME SITS ON TOP OF THE TOWN HALL, WHICH IS 20 M HIGH. HOW HIGH DOES THE COLUMN HAVE TO BE, TO REACH FROM THE FLOOR TO THE DOME?
Thanks.

23.67 M

Dome's height + Town hall height