Hello, uselessjack!
Is there more information?
There are four forms of the parabola, each with its own equation.
I'll derive one of them . . .
Find the equation for the parabola that has its focus on the positive xaxis,
4 units away from the directrix. This is a "vertical" parabola, opening upward.
The focus is at $\displaystyle F(f,0)$
The directrix is: $\displaystyle y = 4$ Code:


*  *
 F
  * +    o     *  
*  (f,0) *
* : *
 * : *
 o V
 (f,2)
 :
 :
   +    +     
4

The equation has the form: .$\displaystyle (xh)^2 \:=\:4p(yk)$
The vertex is at: .$\displaystyle V(f,2)$
The value of $\displaystyle p$ is: .$\displaystyle p \,=\,2$
The equation is: .$\displaystyle (xf)^2 \:=\:4(2)\left(y[2]\right) \quad\Rightarrow\quad (xf)^2 \:=\:8(y+2)$