Ответ:

216m^3-343n^3; 12,492.

Объяснение:

3. (6m-7n)(36m^2+42mn+49n^2)=216m^3+252m^2n+294mn^2-252m^2n-294mn^2-343n^3=216m^3-343n^3;

4. 4,123^2-2,123^2=(4,123-2,123)(4,123+2,123)=2*6,246=12,492.