combining like terms using commutative property

Section 8, Problem 3.
3 + 6a + -4 + -5a --> you can move around like terms like 3 and -4, and combine them using the commutative property, since this is an addition problem.

After combining the like terms: 6a and -5a, and 3 and -4, The problem is simplified to:
-1 + 1a

Final solution: -1 + 1a