13 because you do 18-8 and you get 10 and divide that by 2 and you get 5 then add 5 to 8 and you get 13 and subtract 5 from 18 and you get 13 as well. Hope that helps
For a line you would draw a straight line with arrows on each end and for a ray you would draw a straight line with an arrow on only one end (depends on which way ray is pointing)