It would be 16 more 6th graders that are not in either band or orchestra.
15 6th Graders in band
9 6th Graders in both classes
+18 6th Graders in orchestra
-----
42 6th Graders in either class
100 6th Graders in total
-42 6th Graders in either class
-------
58 6th Graders in neither class
58-42= 16 more 6th Graders that are not in either class
You're welcome :)