We have :
\(C=\left(x-1\right)\left(x^2-x+1\right)\left(x^2+x+1\right)\left(x+1\right)\)
\(\Leftrightarrow C=\left[\left(x-1\right)\left(x^2+x+1\right)\right]\left[\left(x+1\right)\left(x^2-x+1\right)\right]\)
\(\Leftrightarrow C=\left(x^3-1\right)\left(x^3+1\right)\)
\(\Leftrightarrow C=x^6-1\)