The information you given isn't enough- I assume you mean a parabola with vertical line of symmetry, vertex at (0, 20), and such that when x=50, y= 0.
(From {x|-50< x< 50}, {y|0< y< 20}, my first guess was x= 50, y= 20 but then I saw that the vertex was at y= 20 so the parabola must be opening downward.)
Since you only ask about p, I assume you understand that since the vertex is at (0, 20), the equation is (x- 0)^{2}= 4p(y- 20). Now, put x= 50, y= 0 in that and solve for p.