Which equation represents the following situation? the difference of three times a number

Add the two expressions 3.8b - 7 and 2.9b + 9 enter your answer in simplified form

14 pointslet x be the seats in the small van and y the seats in the large van. how many seats does the large van have? a company uses two vans to transport worker from a free parking lot to the workplace between 7: 00 and 9: 00a.m. one van has 9 more seats than the other. the smaller van makes two trips every morning while the larger one makes only one trip. the two vans can transport 69 people, maximum.how many seats does the larger van have?

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?