In order to add the two expressions (-3x+7) and (-6x+9), you combine the like terms, or the terms with the same variable and are raised to the same power. Because -3x and -6x are like terms, you can add those together to result in -9x. Then, because 7 and 9 are like terms, you combine those to get 16. Because -9x and 16 are not like terms, your answer will remain as:
-9x+16
