Simplify the expression: 6a−3a+5b−2b6a - 3a + 5b - 2b6a−3a+5b−2b.
3a+3b3a + 3b3a+3b
9a+7b9a + 7b9a+7b
6ab6ab6ab
3a−3b3a - 3b3a−3b