A=3a^2-6ab+b^2,B=-a^2-5ab-7b^2
2A-3B=2*(3a^2-6ab+b^2)-3*(-a^2-5ab-7b^2)
=6a^2-12ab+2b^2+3a^2+15ab+21b^2
=9a^2+3ab+23b^2