Write an equation of a parabola that opens downward, has a vertex at the origin, and a focus at (0, –1).

y=a(x-h)² + k, vertex(0,0)
(h, k) vertex, h=0, k=0
y=a(x-0)²+0=ax
y=ax²
focus(h, k+ 1/4*a)     focus (0, -1)
k+ 1/4 * a= - 1,
0+1/4 a=-1
1/4a=-1
a=-4
y =ax². y=-4x²