ok, so for parabola a(x-h)^2 +k
abs(a) = 1/(4c) where c is the distance between the focus and directrix.
The distance is 4-1, or 3 (y coord of the focus - y coord of directrix)
so a = 1/(4*3) or 1/12
and the vertex (h,k) is the midpoint of the segment between the focus and directrix, which is (1, 2.5)
the parabola is 1/12*(x-1)^2 + 2.5
