$$(20-a)-6\frac{7}{18}=3\frac{17}{18};$$

Решим уравнение:

$$20 - a - 6\frac{7}{18} = 3\frac{17}{18}$$

$$20 - a = 3\frac{17}{18} + 6\frac{7}{18}$$

$$20 - a = 9\frac{24}{18}$$

$$20 - a = 9 + 1\frac{6}{18}$$

$$20 - a = 10\frac{1}{3}$$

$$a = 20 - 10\frac{1}{3}$$

$$a = 9\frac{2}{3}$$