А)...=(3a^2+3b^2-2a^2-2ab-2b^2)/(3a^3-3b^3)=(a^2-2ab-b^2)/3(a^3-b^3)=(a-b)^2/3(a-b)(a^2+ab+b^2)=(a-b)/3(a^2+ab+b^2)
б)...=(2m^2-m^2-mn+n^2+mn)/(m^3+n^3)=(m^2+n^2)/(m^3+n^3)