Simple..............
you have the endpoint -9 and it goes to +7...
so...
[tex]-9 -8 -7 -6 -5 -4 -3 -2 -1 --\ \textgreater \ \ \textgreater \ 0 1 2 3 4 5 6 7[/tex]
If you count all those points from -9 to +7...
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16...
Thus, your answer +16.
