Find the equation of the parabola with vertex at the origin and focus at $(0, -3)$.

Focus on the **negative $y$-axis** ⇒ the parabola opens **downward** with axis along the $y$-axis. Standard form: $x^2 = -4ay$. $|a| = 3 \Rightarrow x^2 = -12y$. **Trap.** Option D ($y^2 = 12x$) puts the focus on the positive $x$-axis; option C on the negative $x$-axis. Always pick the standard form from the *axis* the focus lies on (vertex at origin pins the rest).