(3a+4b)^2-(3a-b)(2a+b )-2(a-2b)(2b+a)
=9a^2+24ab+16b^2-(6a^2+ab-b^2)-2(a^2-4b^2)
=9a^2+24ab+16b^2-6a^2-ab+b^2-2a^2+8b^2
=a^2+25b^2+23ab
=1/4+25+23/2
=1/4+46/4+25
=47/4+100/4
=147/4