Write an Equation for the Parabola with Vertex at the Origin

Multiple Choice

Write an equation for the parabola with vertex at the origin.
-Through $( 6 , - 12 )$ , opening downward

A) $y = \frac { 1 } { 3 } x ^ { 2 }$
B) $x = - \frac { 1 } { 3 } y ^ { 2 }$
C) $x ^ { 2 } = - \frac { 1 } { 3 }$
D) $x ^ { 2 } = - 3 y$