A particular hydrogen-like ion emits the radiation of frequency \( 3 \times 10^{15} \, \text{Hz} \) when it makes transition from \( n=2 \) to \( n=1 \). The frequency of radiation emitted in transition from \( n=3 \) to \( n=1 \) is \( \dfrac{x}{9} \times 10^{15} \, \text{Hz} \), when \( x = \) ____________.
