Вычислить: \(\frac{1}{\frac{1}{18}-\frac{1}{21}}\).

\[\frac{1}{18}-\frac{1}{21}=\frac{21-18}{378}=\frac{3}{378}=\frac{1}{126}.\]

\[\frac{1}{\frac{1}{18}-\frac{1}{21}}=\frac{1}{\frac{1}{126}}=126.\]

Ответ: 126.