In terms of Rydberg's constant R, the wave number of the first Balmer line is

Wave number $=\frac{1}{\mathrm{\lambda }}=\mathrm{R}\left(\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right)$
For first Balmer line n₁= 2, n₂=3  
 Wave number $=\mathrm{R}\left(\frac{1}{2^2}-\frac{1}{3^2}\right)=\mathrm{R}\left(\frac{9-4}{9\times 4}\right)=\frac{5\mathrm{R}}{36}$