`50 - 2(x + 3) = 30`

`2x + 6 = 20`

`2x = 14`

`x = 7(` Thỏa mãn `)`

Vậy: `x = 7`

Ta có: $A = \dfrac{\left(\dfrac{-5}{7} - \dfrac{7}{9} + \dfrac{9}{11} - \dfrac{11}{13}\right) \cdot \left(3 - \dfrac{3}{4}\right)}{\left(\dfrac{10}{21} + \dfrac{14}{27} - \dfrac{6}{11} + \dfrac{22}{39}\right) : \left(2 - \dfrac{2}{3}\right)}$

$A = \dfrac{\left(\dfrac{-5}{7} - \dfrac{7}{9} + \dfrac{9}{11} - \dfrac{11}{13}\right) \cdot \dfrac{9}{4}}{\left(\dfrac{10}{21} + \dfrac{14}{27} - \dfrac{6}{11} + \dfrac{22}{39}\right) : \dfrac{4}{3}}$

$A = \dfrac{\left(\dfrac{-5}{7} - \dfrac{7}{9} + \dfrac{9}{11} - \dfrac{11}{13}\right) \cdot \dfrac{9}{4}}{\left(\dfrac{10}{21} + \dfrac{14}{27} - \dfrac{6}{11} + \dfrac{22}{39}\right) \cdot \dfrac{3}{4}}$

$A = \dfrac{\left(\dfrac{-5}{7} - \dfrac{7}{9} + \dfrac{9}{11} - \dfrac{11}{13}\right) \cdot \dfrac{9}{4}}{-\dfrac{2}{3} \cdot \left(\dfrac{-5}{7} - \dfrac{7}{9} + \dfrac{9}{11} - \dfrac{11}{13}\right) \cdot \dfrac{3}{4}}$

$A = \dfrac{\dfrac{9}{4}}{-\dfrac{2}{3} \cdot \dfrac{3}{4}}$ $A = \dfrac{\dfrac{9}{4}}{-\dfrac{1}{2}}$

$A = \dfrac{9}{4} \cdot (-2)$

$A = \dfrac{-18}{4}$

$A = -4,5$