(-3)^21+(-3)^20+6*3^19
=(-3)^19*(-3)^2+(-3)^19*(-3)+6*3^19
=(-3)^19*[(-3)^2+(-3)]+6*3^19
=(-3)^19*[9-3]+6*3^19
=(-3)^19*6+6*3^19
=[(-3)^19+3^19]*6
=0*6
=0
(-3)^21+(-3)^20+6*3^19
=(-3)^19*(-3)^2+(-3)^19*(-3)+6*3^19
=(-3)^19*[(-3)^2+(-3)]+6*3^19
=(-3)^19*[9-3]+6*3^19
=(-3)^19*6+6*3^19
=[(-3)^19+3^19]*6
=0*6
=0