(2a-b)(b+2a)+3(2a-b)^2+(-3a)(4a-3b)-(3a-b)(b-2a)
=4a^2-b^2+6a^2-6ab+3b^2-12a^2+9ab+6a^2-5ab+b^2
=4a^2-2ab+3b^2
=4+2+12=18