$(3x^{4}+7x - 4x^{3})+(8x - 6x^{4}+2x^{3})-(8x^{4}+8x)$

Explanation:

Step1: Remove parentheses

$(3x^{4}+7x - 4x^{3})+(8x - 6x^{4}+2x^{3})-(8x^{4}+8x)=3x^{4}+7x - 4x^{3}+8x - 6x^{4}+2x^{3}-8x^{4}-8x$

Step2: Combine like - terms of $x^{4}$

$3x^{4}-6x^{4}-8x^{4}=(3 - 6 - 8)x^{4}=-11x^{4}$

Step3: Combine like - terms of $x^{3}$

$-4x^{3}+2x^{3}=(-4 + 2)x^{3}=-2x^{3}$

Step4: Combine like - terms of $x$

$7x+8x-8x=(7 + 8 - 8)x=7x$

Answer:

$-11x^{4}-2x^{3}+7x$