-4m².(n²-m²)+3n².(m²-n²)

-4m².(n²-m²)+3n².(m²-n²) =

Раскроем скобки:

$$-4m^2 \cdot n^2 + 4m^2 \cdot m^2 + 3n^2 \cdot m^2 - 3n^2 \cdot n^2 =$$

$$-4m^2n^2 + 4m^4 + 3m^2n^2 - 3n^4 =$$

Приведем подобные члены:

$$4m^4 - 4m^2n^2 + 3m^2n^2 - 3n^4 =$$

$$4m^4 - m^2n^2 - 3n^4$$

Ответ: 4m⁴-m²n²-3n⁴