z³+21+3z+7z²=z³+7z²+21+3z=z²(z+7)+3(7+z)=(z+7)(z²+3)
z³+21+3z+7z²= (z³+7z²)+(21+3z)= z²(z+7)+3(7+z)=(z+7)(z²+3).
z³+21+3z+7z²=z³+7z²+21+3z=z²(z+7)+3(7+z)=(z+7)(z²+3)
z³+21+3z+7z²= (z³+7z²)+(21+3z)= z²(z+7)+3(7+z)=(z+7)(z²+3).