When you solve this by using an equation, you would get 77=65+0.80m (0.80 m because slope is 0.80 and m is mileage you multiply by). You would subtract 64 from both sides, so 12=0.80m and then you divide 0.80 by 12 to get m by itself to get 15. 15 miles is when both companies cost the same, so anything above 15.
