They are intersecting at 0,0. One square on that chart is 2 units. You can see this since it goes -10 then skips one then goes to -6. The one in between is -8. On every side of the intersection there is a 2 or -2. 2 - 2 or -2 + 2, depending if it's positive or negative, is 0. So the lines are intersecting at 0,0. Hope this helps! :)
