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

Wave number $=\frac{1}{\lambda}=R\left(\frac{1}{n_1^2}-\frac{1}{n_2^2}\right)$

For first Balmer line $n_1=2, n_2=3$

∴ Wave number $=R\left(\frac{1}{2^2}-\frac{1}{3^2}\right)=R\left(\frac{9-4}{9 \times 4}\right)=\frac{5 R}{36}$