3. б) $$(3,15+2,1)\cdot (2\frac{13}{28}-1\frac{3}{14})$$

$$\begin{aligned} &3,15 + 2,1 = 5,25 \\ &2\frac{13}{28} - 1\frac{3}{14} = 2\frac{13}{28} - 1\frac{6}{28} = 1\frac{7}{28} = 1\frac{1}{4} = \frac{1 \cdot 4 + 1}{4} = \frac{5}{4} \\ &5,25 \cdot \frac{5}{4} = 5\frac{25}{100} \cdot \frac{5}{4} = 5\frac{1}{4} \cdot \frac{5}{4} = \frac{5 \cdot 4 + 1}{4} \cdot \frac{5}{4} = \frac{21}{4} \cdot \frac{5}{4} = \frac{21 \cdot 5}{4 \cdot 4} = \frac{105}{16} = 6\frac{9}{16} \end{aligned}$$ Ответ: $$\bf{6\frac{9}{16}}$$