Multiply the quantities of each by the prices, then add it all together to find total spent. Subtract total spent from what he has to spend to find the change:
3 shirts x 17.25 = 51.75
5 jeans x 27.75 = 138.75
Total pent : 62.50 + 51.75 + 138.75 = $253.00
Change : 300 - 253 = $47.00