(a+b)²-9a²
=(a+b+3a)(a+b-3a)
=(4a+b)(b-2a)
9(m+n)²-(m-n)²
=(3m+3n-m+n)(3m+3n+m-n)
=(2m+4n)(4m+2n)
=4(m+2n)(2m+n)