(3x²-2y²)(-2y²-3x²)+3(x²-y²)(x²+y²)
= -(3x²-2y²)(3x²+2y²)+3(x²-y²)(x²+y²)
=-(9x⁴-4y⁴)+3(x⁴-y⁴)
=-9x⁴+4y⁴+3x⁴-3y⁴
=-6x⁴+y⁴