first you look at the graph try to make a educated guess on were the x and y could be which is close to 8 and 10 soo you subtract it which is 2 when you get the 2 you will add that with 6 to make 8!!!
8 tortillas as per the graph Follow the X line to the right for 4 orders, now go up the Y line to the number of corn tortillas needed indicated by the dot
