Directions: Simplify by combining like terms in each of the following expressions.
1. 1s2 + 10s2 - 21st =
2. 5st - st - t2 =
3. x^{2} + x^{2} + x^{3} =
4. 6x - 4x + 2=
5. 9x - 3s - 2t=
6. 87x + 8x - 1 =
7. 3x + 6x - 2 =
8. 8s - 4s + 3s =
9. 12 + x - 11x =
10. 12x + 24x - 3x - 4 =
11. 14t + 8t - 12t =
12. 17g + 14g - 6g + g =
13. 13 + 12 - 12 + 13x =
14. 17s - 3s + 3s =
15. 14t - 12t + t =