y represents the total cost x represents the number of miles towed (since it costs 3.50 per one mile, that's why the variable x needs to be here because we don't know how many miles we'll have yet)
In 4 hours they will be 400 miles apart 4 times 55 is 220 and 4 time 45 is 180 since there going in opposer directions add the miles together and it equals 400 so it takes 4 hours