Algebra II question helpl?!?

Question

Which of the following is represented by a parabola with vertex at the origin and focus (-3,0)?

A) x = 1/12y^2

B) y = - 1/12x^2

C) y = 1/12x^2

D) x = - 1/12y^2

Please help me I have no idea D;

Answer 1

Given vertex = (0,0) and focus = (-3,0)

Let focus (-3,0) = (-a,0)

The equation of parabola with vertex (0,0) and focus (-a,0) is y^2 = -4ax------------(1)

Substitute a=3 in (1)

y^2 = - 4(3)x

y^2 = -12x

Divide each side by -12

-1/12y^2 = x

Therefore x = -1/12y^2

The answer is option D