Add the two expressions 3w + 4 and 3w - 9

3w+4 nothing can be done

All together the expression would look like (3w+4) + (3w-9). Since you can add like numbers, 3w, 3w+3w would be 6w. Then, - 9 + 4 is - 5. So, now you have your two sums, 6w and - 5. So, the sum of the two expressions would be (6w - 5)