Simplify $\left(-2x^2-x-5\right)+\left(6x^2-9x-8\right)$(−2x2−x−5)+(6x2−9x−8).
Simplify $\left(-8x^2-1\right)+\left(7x^2+8x\right)$(−8x2−1)+(7x2+8x).
Simplify $\left(5x^3-7x^2-1\right)+\left(5x^3-6x^2+9x\right)$(5x3−7x2−1)+(5x3−6x2+9x).
Simplify $\left(2x^2-3x-9\right)-\left(-6x^2+6x-2\right)$(2x2−3x−9)−(−6x2+6x−2).