143. Find the canonical equation of the parabola and the equation of its directrix, given that the vertex of the parabola lies at the origin, and the focus has coordinates $(0; -3)$.
Solution. According to the condition, the focus of the parabola is located on the negative half-axis $O y$, i.e., its equation has the form $x^{2}=-2 p y$. Since $-p / 2=-3$, then $p=6$, from which $2 p=12$. Therefore, the equation of the parabola is $x^{2}=-12 p y$, and the equation of the directrix is $y=3$ or $y-3=0$.