For this expression (3m - 2n)³ = (9m² -12mn + 4n²)
1)Let's remember the difference of two cubes
(a -b)³ = a³ -3a²b +3ab²-b³
(3m - 2n)³ = (3m)³ -3 (3m)²(-2n) +3(3m)(-2n)²-(2n)³ = 27m³ -54m²n +36mn²-8n³
27m³ -54m²n +36mn²-8n³ = 9m² -12mn + 4n²
2) Combining Like terms:
27m³ -54m²n +36mn²-8n³ -9m² +12mn+4n² =0
Since we can't get a simpler version of it. Let's keep with that.