$B=\Bigg(\dfrac{5}{7}.0,6-5:3\dfrac{1}{2}\Bigg).(40\%-1,4).(-2)^3\\=\Bigg(\dfrac{5}{7}.\dfrac{3}{5}-5:\dfrac{7}{2}\Bigg)\Bigg(\dfrac{2}{5}-\dfrac{7}{5}\Bigg).(-8)\\=\Bigg(\dfrac{3}{7}-\dfrac{10}{7}\Bigg).\dfrac{2-7}{5}.(-8)\\=\dfrac{3-10}{7}.(-1).(-8)\\=(-1).8\\=-8\\\text{b) }\dfrac{1}{2}x+\dfrac{3}{5}(x-2)=3\\⇔\dfrac{1}{2}x+\dfrac{3}{5}x-\dfrac{6}{5}=3\\⇔\dfrac{11}{10}x=3+\dfrac{6}{5}\\⇔\dfrac{11}{10}x=\dfrac{21}{5}\\⇔x=\dfrac{21}{5}:\dfrac{11}{10}\\⇔x=\dfrac{21}{5}.\dfrac{10}{11}\\⇔x=\dfrac{42}{11}$