Answer:
Explanation:
The missing options are:
![a.\ 15 + 6\\b.\ -6 + (-15)\\c.\ 15 + (-6)\\d.\ 6 + (-15)](https://img.qammunity.org/2021/formulas/mathematics/middle-school/vub55oylgw6a0ou922a1ic8kdnmffs52qa.png)
For this exercise it is important to remember the mulitplication of signs:
![(+)(+)=+\\(-)(-)=+\\(-)(+)=-\\(+)(-)=-](https://img.qammunity.org/2021/formulas/mathematics/middle-school/avp11hyx9fm29rgb2ec5wtv3gh2ogaa0ln.png)
By definition, the sum is the result of an addition.
Therefore, you need to add the numbers given in the exercise (-15 and 6) in order to find its sum. This is:
(Since the greatest number has a negative sign, the sum is negative).
Knowing this, you can check each option to find the expression whose sum is -9:
![a.\ 15 + 6=21\\\\b.\ -6 + (-15)=-6-15=-21\\\\c.\ 15 + (-6)=15-6=9\\\\d.\ 6 + (-15)=6-15=-9](https://img.qammunity.org/2021/formulas/mathematics/middle-school/pzhz6wpsg73ogwsd4m8nnskreg1tmp2im4.png)
The answer is the option "d".