\[6x - 8y\ \ при\ \ x = \frac{2}{3};\ \ y = \frac{5}{8}.\]
\[6 \cdot \frac{2}{3} - 8 \cdot \frac{5}{8} = \frac{6 \cdot 2}{3} - \frac{8 \cdot 5}{8} = 4 - 5 = - 1.\]