I don't know the answer to this question. I tried -1 but it wasn't right. Help!

QUESTION 7 S1, S2, S3 and $4 are UIUC students who just graduated. They each have a poster of Dolly Parton that they are willing to sell. B1, B2, B3 and B4 are new students at UIUC and want to buy Dolly Parton posters for their rooms. The tables below give the minimum selling prices and maximum buying prices for each student. Minimum Selling Prices Maximum Buying Prices S1 $10 B1 X+$9 S2 $15 B2 X+$14 S3 $20 B3 X+$16 S4 $25 B4 X+$26 What is the minimum possible value of X such that all 4 posters are exchanged at the market-clearing price? Note: Do not include the dollar sign