12) (1 3/13 - 2 3/4) * 26

$$\left(1 \frac{3}{13} - 2 \frac{3}{4}\right) \cdot 26 = \left(\frac{16}{13} - \frac{11}{4}\right) \cdot 26 = \left(\frac{64}{52} - \frac{143}{52}\right) \cdot 26 = -\frac{79}{52} \cdot 26 = -\frac{79}{2} = -39,5$$

Ответ: $$\bf{-39,5}$$