(a-2b)^2-3(a+b)(a-b)+(3a^2+2ab-6b^2)
=a^2-4ab+4b^2-3a^2+3b^2+3a^2+2ab-6b^2
=a^2-2ab+b^2
a^2-2ab+b^2
=1^2-2*1*2+2^2
=1