Two cells of same emf E, but different internal resistance and are connected to an external…

Two cells of same emf E, but different internal resistance $r_1$ and $r_2$ are connected to an external resistance R as shown in Figure 5 given below. The voltmeter V reads zero. Obtain an expression for R in terms of $r_1$ and $r_2$. (Assume that the voltmeter V is of infinite resistance). (Application)