Which is the equation of a parabola with vertex (0, 0), that opens downward and has a focal width of 6?

Vertex: V=(0,0)=(h,k)→h=0, k=0
Opens downward:
(x-h)^2=4p(y-k)
Width focal: p=-6<0 (donwnward)

Replacing h=0, k=0 and p=-6 in the equation above:
(x-0)^2=4(-6)(y-0)
x^2=-24y

Answer: The equation of the parabola is  x^2=-24y