Sarah took the advertising department from her company on a round trip to meet with a potential client. Including Sarah, a total of 17 people took the trip. She was able to purchase coach tickets for ​$270 and first-class tickets for ​$1140. She used her total budget for airfare for the​ trip, which was ​$12,420 How many first-class tickets did she​ buy? How many coach tickets did she​ buy?