The parabola lies midway between the focus and directrix line, which is (2,(8+2)/2)=(2,5). This is the vertex. But the directrix line is higher than the focus so the parabola is inverted. The distance between the directrix line and the parabola is 4a=3, so a=-1/12 (minus inverts the parabola).
This can be written 12(y-5)=-(x-2)^2 or 12y=-x^2+4x+56 or x^2+12y-4x-56=0.