Let's consider the horizontal parabola with vertex at a generic point $$A(x_0,y_0)$$.
In this case the focus is on $$F(x_0+\dfrac{p}{2},y_0)$$ and the generator line is $$x=x_0-\dfrac{p}{2}$$.
The equation of the parabola under these conditions is $$$(y-y_0)^2=2p(x-x_0)$$$
The equation of the parabola which focus point is at $$F(-2,4)$$ and the vertex point at $$A(3,4)$$.
Identify $$A(x_0,y_0)$$ with $$A(3,4)$$ and $$F(x_0+\dfrac{p}{2},y_0)$$ with $$F(-2,4)$$. We obtain $$x_0=3$$ and $$y_0=4$$.
By analyzing the focus and the generic equation we know that $$$x_0+\dfrac{p}{2}=3+\dfrac{p}{2}=-2$$$, then $$\dfrac{p}{2}=5$$ and we obtain the parameter value $$p=10$$.
Substituting into the equation $$(y-y_0)^2=2p(x-x_0)$$ we obtain $$$(y-4)^2=20(x-3)$$$