16/7 would convert to 2 2/7. When you add -9 + 2 2/7 you add whole numbers first then fractions. -9 + 2 equals -7. From here you need to fill in the left over fraction by taking from the -7 itself. That's where you get your 5/7 froms, because you are adding 2/7 and 5/7 to get one, which in turn that 1 subtracts from 7 to get -6.
