\[\boxed{\mathbf{2.578}}\]
\[\textbf{а)}\ 6,76 \cdot \frac{5}{13} = \frac{676 \cdot 5}{100 \cdot 13} =\]
\[= \frac{52 \cdot 5}{100} = \frac{260}{100} = 2,6\]
\[\textbf{б)}\ 9,8\ :1\frac{3}{4} = \frac{98}{10}\ :\frac{7}{4} = \frac{49}{5} \cdot \frac{4}{7} =\]
\[= \frac{7 \cdot 4}{5} = \frac{28}{5} = 5,6\]
\[\textbf{в)}\ 8,4 \cdot 1\frac{3}{7} = \frac{84}{10} \cdot \frac{10}{7} = 12\]
\[\textbf{г)}\ 14,3\ :1\frac{5}{8} = \frac{143}{10}\ :\frac{13}{8} =\]
\[= \frac{143}{10} \cdot \frac{8}{13} = \frac{11 \cdot 8}{10} = \frac{88}{10} = 8,8\]