Trigonometric Ratio and Identites

If $\tan \left( {{\pi \over 9}} \right),x,\tan \left( {{{7\pi } \over {18}}} \right)$ are in arithmetic progression and $\tan \left( {{\pi \over 9}} \right),y,\tan \left( {{{5\pi } \over {18}}} \right)$ are also in arithmetic progression, then $|x - 2y|$ is equal to :