What is the equation of a parabola with a directrix x = 8 and focus (-8, 0) ?

Given the focus and the directrix of the parabola and the information that the length from the vertex to the parabola is equal to the length from the vertex to the focus, we can say that the vertex is at (0,0) and is opening to the left. Hence the general formula is y2 = - 4ax. a is the length said above equal to 8. hence the equation is y2 = - 32x