3) 5/7 x + 2 1/7 = 3 3/28 – 4/7 x;

3) $$ \frac{5}{7}x + 2 \frac{1}{7} = 3 \frac{3}{28} - \frac{4}{7}x $$

$$ \frac{5}{7}x + \frac{4}{7}x = \frac{87}{28} - \frac{15}{7} $$

$$ \frac{9}{7}x = \frac{87}{28} - \frac{15 \cdot 4}{7 \cdot 4} $$

$$ \frac{9}{7}x = \frac{87}{28} - \frac{60}{28} $$

$$ \frac{9}{7}x = \frac{27}{28} $$

$$ x = \frac{27}{28} : \frac{9}{7} $$

$$ x = \frac{27}{28} \cdot \frac{7}{9} $$

$$ x = \frac{3}{4} $$

Ответ: x = 0,75