[tex]\frac{2}{7}(1,4x +21) = -\frac{3}{5}x\\\frac{2}{7}(\frac{14}{10}x +21) = -\frac{3}{5}x\\\\\frac{2}{7} \cdot \frac{14}{10}x + \frac{2}{7} \cdot 21 = -\frac{3}{5}x\\\frac{4}{10}x + 6 = -\frac{3}{5}x\\\frac{2}{5}x + 6 = -\frac{3}{5}x / -\frac{2}{5}x\\6 = -\frac{3}{5}x -\frac{2}{5}x = -x\\-x = 6 /\cdot (-1)\\x = -6[/tex]
