Выполним вычисление по действиям:
- $$1\frac{1}{4} = \frac{5}{4}$$
- $$2\frac{4}{15} = \frac{34}{15}$$
- $$2\frac{1}{8} = \frac{17}{8}$$
- $$\frac{5}{17} \cdot \frac{34}{15} = \frac{5 \cdot 34}{17 \cdot 15} = \frac{1 \cdot 2}{1 \cdot 3} = \frac{2}{3}$$
- $$\frac{17}{8} : \frac{51}{56} = \frac{17}{8} \cdot \frac{56}{51} = \frac{17 \cdot 56}{8 \cdot 51} = \frac{1 \cdot 7}{1 \cdot 3} = \frac{7}{3}$$
- $$\frac{5}{4} + \frac{2}{3} = \frac{5 \cdot 3 + 2 \cdot 4}{12} = \frac{15 + 8}{12} = \frac{23}{12}$$
- $$\frac{23}{12} - \frac{7}{3} = \frac{23 - 7 \cdot 4}{12} = \frac{23 - 28}{12} = \frac{-5}{12}$$
Ответ: $$-\frac{5}{12}$$