[(x+1)(x+6)][(x+2)(x+3)]-15x^2
=(x^2+5x+6)(x^2+7x+6)-15x^2
令x^2+6=a

原式變成
a^2 + 12xa +35x^2 -15x^2
=a^2 + 12xa +20x^2
=(a+10x)(a+2x)
=(x^2 + 10x + 6)(x^2 + 2x + 6)

Case closed.