Its 90 because 90 divided by 10 simplifies down to 9, and 9+8=17
No they cant not share the same y intercept.
(4,10) because there are 2 points with an x value of 9, 2 points with a y value of 5, and there should be 2 with an x value of 4, and 2 with a y value of 10.