Given Vertex $(0,0)$ and Focus $(3,0)$
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 = $(3,0) \rightarrow a = 3$
Therefore the equation of the parabola is $y^2 = 4 \times 3x = 12x$