As a fundraiser, a school club is selling posters. The printer charges a $23 set-up fee, plus $0.75 for each poster. Then the cost y in dollars to print x posters is given by the linear equation y=0.75x+23. The printing of 50 posters cost $60.50. The printing of 100 posters costs $98.00. Find the number of posters x if the printer billed the club for the costs of $173.00.