Given Vertex $(0,0)$ and Focus $(-2,0)$
Since the vertex is the origin and the focus lies on the positive x-axis, $\rightarrow $ x-axis is the axis of the parabola.
The parabola is of the form $y^2=-4ax$.
Since Focus = $(-3,0) \rightarrow a = 2$
Therefore the equation of the parabola is $y^2 =-4 \times 2x = -8x$