\(A=n^4-n^3-6n^2+7n-21\)
\(A=n^4-3n^3+2n^3-6n^2+7n-21\)
\(A=n^3\left(n-3\right)+2n^2\left(n-3\right)+7\left(n-3\right)\)
\(A=\left(n^3+2n^2+7\right)\left(n-3\right)\)
\(A=n^4-n^3-6n^2+7n-21\)
\(A=n^4-3n^3+2n^3-6n^2+7n-21\)
\(A=n^3\left(n-3\right)+2n^2\left(n-3\right)+7\left(n-3\right)\)
\(A=\left(n^3+2n^2+7\right)\left(n-3\right)\)