(20 - a) - 6\(\frac{7}{18}\) = 3\(\frac{17}{18}\)
20 - a = 3\(\frac{17}{18}\) + 6\(\frac{7}{18}\)
20 - a = 9\(\frac{24}{18}\) = 10\(\frac{6}{18}\) = 10\(\frac{1}{3}\)
a = 20 - 10\(\frac{1}{3}\)
a = 9\(\frac{2}{3}\)