Zoom International Math League

The question states that there are 5 pennies and two match sticks and it asks to find the number of ways that you can put them in a row, essentially making it a stars and bars problem. The answer was 7 choose 2 and was found by 5+3-1 choose 3-1. I don't get why a three is on the top and bottom when the question says two. Also something is the -1 on the bottom which I don't understand why is there.