You multiply 0.75 into everything in the parenthesis. 0.75 * 3.5a = 2.625a and 0.75 * -6b = -4.5b. Then you would add the two answers: 2.625a + (- 4.5b ). Since there are two different variables, you cannot add the two. However, if the variables were the same, you could add them and get -1.875 (a or b).