You have to combine like terms, so the variable (x, y, s, d, c....) and the exponents must be the same in order to combine them.
For example:
x² + x³ Since they don't have the same exponent, you can't combine them
y² + 3y² = 4y²
23x + x = 24x
4. 2s² + 1 + s² - 2s + 1 You can rearrange it if it makes it easier
2s² + s² - 2s + 1 + 1 = 3s² - 2s + 2
5. 5t² - 2t - 1 - (3t² - 5t + 7) Distribute/multiply the - to (3t² - 5t + 7)
5t² - 2t - 1 - 3t² + 5t - 7 = 2t² + 3t - 8
Do the same for #9 and #10, and you should get:
9. 2k² + 5k - 9
10. 6y³ - 7y² - 6y - 12