Rules in adding integers :
1. Same signs (+)
take the sum of the integers and it is a positive integer
2. Same signs (-)
take the sum of the integers and it is a negative integer
3. Different signs (+ and -)
Take the difference of the integers then follow the sign of the larger number
From the given :
You can work with the integers with the same sign first then proceed with the integers with different signs.
Add integers with the same signs
(-18) + (-15) = -33 (Rule #2)
Now we have :
Using Rule #3
Take the difference :
33 - 10 = 23
Then the sign will follow the sign of the larger number which is -33
So the answer is -23