V=number of vans and B=number of buses.
9V+17B=88. So V=(88-17B)/9=(90-2-(18-1)B)/9=10-2/9-2B+B/9=10-2B+(B-2)/9.
If (B-2)/9 is an integer then we get positive integer values for B and V.
If B=2, V=6. No other value of B is possible.
So 6 vans will carry 54 people and 2 buses will carry 34, total 88 people.
The question says “at least” 88 people, so the minimum requirements are 6 vans and 2 buses.