a) $$8\frac{3}{4} \cdot 1\frac{3}{14} = \frac{35}{4} \cdot \frac{17}{14} = \frac{5 \cdot 17}{4 \cdot 2} = \frac{85}{8} = 10\frac{5}{8}$$
б) $$\frac{3,5+1,9}{2,4} = \frac{5,4}{2,4} = \frac{54}{24} = \frac{9}{4} = 2\frac{1}{4} = 2,25$$
Ответ: a) $$10\frac{5}{8}$$, б) $$2,25$$