(3m²-n²)²-(m²-3n²)²
=[(3m²-n²)+(m²-3n²)][(3m²-n²)-(m²-3n²)]
=(4m²-4n²)(2m²+2n²)
=8(m²-n²)(m²+n²)
=8(m+n)(m-n)(m²+n²)