a) \(\left[2x\left(x^2+4x+3\right)-8\left(x^2+4x+3\right)\right]:\left(x^2+4x+3\right)\)
\(=\left[\left(x^2+4x+3\right)\left(2x-8\right)\right]:\left(x^2+4x+3\right)=2x-8\)
b) \(\left[x^2\left(x-3\right)-6x\left(x-3\right)+10\left(x-3\right)\right]:\left(x-3\right)\)
\(=\left[\left(x-3\right)\left(x^2-6x+10\right)\right]:\left(x-3\right)=x^2-6x+10\)