They intersect where F(x,y) = S(x,y).

i.e.

(s, -s² + 4s)=(8 -4*t, t)

i.e.

s = 8 - 4*t (1)

t = -s² + 4s (2)

These are a pair of simultaneous equations, so you should be able to solve them. (Hint: Start by substituting equation (2) into equation(1), and using the quadratic formula to solve for s. Once you've done that, substitute your values for s into equation (1) or (2) to get values for t. Then use these values of s and t in the graph equations to find the points.