3(1/3x²-y²)-2(3y²-2x²)
=x²-3y²-6y²+4x²
=5x²-9y²
=5×(-2)²-9×(-1/3)²
=5×4-9×1/9
=20-1
=19
a²-b²
=(a²-2ab)-(b²-2ab)
=-4-5
=-9