Answer
$${y^2} = - 16x$$
Work Step by Step
$$\eqalign{
& {\text{The focus at }}\left( {3,0} \right){\text{ and Vertex }}\left( {0,0} \right) \cr
& {\text{Then the parabola has the equation }}{y^2} = 4px \cr
& {\text{With focus }}\left( {p,0} \right) \cr
& \left( {3,0} \right)\,\, \Rightarrow \,\,\,\,\,p = 3 \cr
& {\text{The equation of the parabola is:}} \cr
& {y^2} = 4\left( 3 \right)x \cr
& {y^2} = 12x \cr} $$
