For the Hydrogen spectrum, last line of the L series has frequency $V_1$ , last line of Lyman ${\mathrm{He}}^{+}$ ion has frequency ${\mathrm{V}}_2$ and last line of B series of He+ ion has frequency $V_3$ then:

Given,

Frequency of Last line of the L series  = $V_1$

Frequency of last line of Lyman He+ ion has frequency =  ${\mathrm{V}}_2$

As a result 

Frequency of last line of B series of He+ ion ,$V_3$ is,
 

$\begin{array}{l}\frac{{\mathrm{v}}_1}{C}=R_{\mathrm{H}}\left[\frac{1}{1^2}-\frac{1}{{\mathrm{\infty }}^2}\right];\frac{{\mathrm{v}}_2}{C}=R_{\mathrm{H}}\times 2^2\left[\frac{1}{1^2}-\frac{1}{{\mathrm{\infty }}^2}\right]\\ \frac{{\mathrm{v}}_3}{C}=R_{\mathrm{H}}\times 2^2\left[\frac{1}{2^2}-\frac{1}{{\mathrm{\infty }}^2}\right]\end{array}$