In this situation, you would be combining like terms.
What this means is that you solve for the terms that are the same
Solve:
-5a -12a + 2b - 3b
-5a and -12a are like terms, so solve them by combining them
-17a + 2b - 3b
You would also see that 2b and - 3b are the same term, so solve them as well.
-17a - b
There are no more like terms, so your answer would be -17a - b
Answer: -17a - b