a,
\(B=\left(\dfrac{5}{7}\cdot0,6-5:\dfrac{7}{2}\right)\cdot\left(40\%-1,4\right)\cdot\left(-2\right)^3\\ =\left(\dfrac{5}{7}\cdot\dfrac{3}{5}-5\cdot\dfrac{2}{7}\right)\cdot\left(\dfrac{2}{5}-\dfrac{7}{5}\right)\cdot\left(-8\right)\\ =\left(\dfrac{3}{7}-\dfrac{10}{7}\right)\cdot\left(-1\right)\cdot\left(-8\right)\\ =\left(-1\right)\cdot\left(-1\right)\cdot\left(-8\right)\\ =\left(-8\right)\)
b,
\(\dfrac{1}{2}\cdot x+\dfrac{3}{5}\cdot\left(x-2\right)=3\\ \dfrac{1}{2}x+\dfrac{3}{5}x-\dfrac{6}{5}=3\\ x\left(\dfrac{1}{2}+\dfrac{3}{5}\right)=3+\dfrac{6}{5}\\ x\cdot\dfrac{11}{10}=\dfrac{21}{5}\\ x=\dfrac{21}{5}:\dfrac{11}{10}\\ x=\dfrac{42}{11}\\ Vậy......\)