Answer:
-9
Step-by-step explanation:
If you have a problem that is plus a negative, you can get rid of the plus sign and move the minus sign to be the operation. In this problem, it would make the equation 6 - 15. We know that 6 + ? will equal 15 just as much as 6 - 15 = ?
6 + 9 = 15 therefore 6 - 15 must equal -9.
