a) \(x^2\left(x+1\right)-\left(x+3\right)\left(x^2-3x+9\right)\)
\(=x^3+x^2-x^3-9\)
\(=x^2-9\)
\(=\left(x-3\right)\left(x+3\right)\)
b) \(2\left(x^2-1\right)-\left(x-1\right)^2-\left(x+1\right)^2\)
\(=2\left(x-1\right)\left(x+1\right)-\left(x-1\right)^2-\left(x+1\right)^2\)
\(=-\left[\left(x-1\right)^2-2\left(x-1\right)\left(x+1\right)+\left(x+1\right)^2\right]\)
\(=-\left(x-1-x-1\right)^2\)
\(=-2^2\)
\(=-4\)