次数の低い文字で整理する！　です。
\(x^{3}\)-(2a-1)\(x^{2}\)-2(a-1)\(x\)+2
=\(x^{3}\)-2a\(x^{2}\)+\(x^{2}\)-2a\(x\)+2\(x\)+2
=(-2\(x^{2}\)-2\(x\))a+(\(x^{3}\)+\(x^{2}\)+2\(x\)+2)
=-2\(x\)(\(x\)+1)a+{\(x^{2}\)(\(x\)+1)+2(\(x\)+1)}
=-2\(x\)(\(x\)+1)a+(\(x\)+1)(\(x^{2}\)+2)
=(\(x\)+1)(-2a\(x\)+\(x^{2}\)+2)
=(\(x\)+1)(\(x^{2}\)-2a\(x\)+2)