Answer
$ y=-\dfrac {x^{2}}{3}$
Work Step by Step
If focus of parabola is $F(p,0)$ this parabola has a horizontal axis
If focus of parabola is $F(0,p)$ this parabola has a vertical axis
In this case parabola has vertical axis
$\Rightarrow x^{2}=4py=4\times \left( \dfrac {-3}{4}\right) \times y=-3y$
$\Rightarrow y=-\dfrac {x^{2}}{3}$
