0,8(8а-14) = 0,6(6а-8) при а= -3 1/12

Может там не "=" а "-" ?

$0,8(8а-14) - 0,6(6а-8) = \\ = 6.4a - 11.2 - 3.6a + 4.8 = \\ = 2.8a - 6.4 = 2\frac{4}{5} \times (-3 \frac{1}{12}) - 6 \frac{2}{5} = \\ = \frac{14}{5} \times ( - \frac{37}{12}) - 6 \frac{2}{5} = \\ = - \frac{7 \times 37}{5 \times 6} - 6 \frac{2}{5} = \frac{259}{30} - 6 \frac{2}{5} = \\ = 8 \frac{19}{30} - 6 \frac{2}{5} = 2 \frac{19 - 12}{30} = \\ = 2 \frac{7}{30} . \\$