The equation is y^2=-4ax
find something that when you multiply it by 4, the answer is 16
a = 4
y^2 = -4(4)x
y^2 = -16x (it's given)
'a' is always the distance from Focus to Vertex and from Vertex to Directrix
Since the equation is y^2 = -4ax it is opens to the left and the vertex is (0,0)
the Directrix is 4
or (4,0) = directrix
