(7x^3 + 2x - 1) + (8x^2 - 6 + 2x - x^3)
= 6x^3 + 8x^2 + 4x - 7
(-3x^4 + x^6 - 9x^5 + 2x^2 - 7) - (-2x^5 + x - 4x^2 - x^4 + 12)
= x^6 - 7x^5 - 2x^4 + 6x^2 - x - 19
correct.
You can easily check your results for yourself at wolframalpha.com
= 6x^3 + 8x^2 + 4x - 7
(-3x^4 + x^6 - 9x^5 + 2x^2 - 7) - (-2x^5 + x - 4x^2 - x^4 + 12)
= x^6 - 7x^5 - 2x^4 + 6x^2 - x - 19
You can easily check your results for yourself at wolframalpha.com