121(a^2-2ab+b^2)-169(a^2+2ab-b^2)
=[11(a-b)]^2-[13(a+b)]^2
=(11a-11b+13a+13b)(11a-11b-13a-13b)
=(24a+2b)(-2a-24b)
=-4(12a+b)(a+12b)