\[- c + c^{7} = - c\left( 1 - c^{6} \right) =\]
\[= - c\left( 1 - \left( c^{3} \right)^{2} \right) =\]
\[= - c\left( 1 - c^{3} \right)\left( 1 + c^{3} \right) =\]
\[= c(c - 1)\left( 1 + c + c^{2} \right)(1 + c)\left( c^{2} - c + 1 \right).\]