36-5(m+n)-(m-n)的平方.应该如何因式分解?

36-5(m+n)-(m-n)^2
=[9+(m-n)][4-(m-n)]
=-(m-n+9)(m-n-4)
(x+1)(x+2)(x+3)(x+6)-3x^2
=[(x+1)(x+6)][(x+2)(x+3)]-3x^2
=[(x^2+6)+7x][(x^2+6)+5x]-3x^2
=(x^2+6)^2+12x(x^2+6)+35x^2-3x^2
=(x^2+6)^2+12x(x^2+6)+32x^2
=(x^2+6+4x)(x^2+6+8x)