It's more easy, from the first question you already know the equations, you want to find S, so you have to find those two equations intersection, which means:
1/2x-1=-2x+9
1/2x+2x=9+1
2.5x=10
x=4
then put x=4 in to any of the equation: y=-2x+9. x=4
y=-2x4+9
y=-8+9
y=1
so the coordinate of S is (4,1)