If sum of the first \(21\) terms of the series \( \displaystyle \log_{9^{1/2}} x + \log_{9^{1/3}} x + \log_{9^{1/4}} x + \dots, \), where \( x>0 \), is \(504\), then \(x\) is equal to:
\( 81 \)
\( 7 \)
\( 243 \)
\( 9 \)