Problem: (5x^4 – 3x^3 + 6x) – (3x^3 + 11x^2 – 8x)

5x^4-6x^3 -11x^2 +14x

Combine the like terms by adding the coefficients,
->5x^4 can't add anything so leave it there,
->-3x^3 can add -3x^3 which gives you -6x^3,
->We can't do anything with -11x^2 so leave it there as well,
->6x add to -(-8x) gives you 14x,
You will get for your final answer is 5x^4-6x^3-11x^2+14x.

Sorry i meant b in the other question