a^{42} -14a^{7} -(a^{6} -14)=a^{7} (a^{6} -14)-(a^{6} -14)=(a^{6} -14)(a^{7}-(a^{7}-1))=(a^{6} -14)(a^{7}-a^{7}+1)=a^{6} -14
