Let first watt meter be connected between R and B

$\begin{align*} \therefore \omega _1 &= V_{RB}.I_R \cos (V_{RB}-I_R)\\ &=V_{RB}.I_R \cos (30-\phi)\\ &=V_L I_L \cos (30-\phi)...(1)\\ \end{align*}$

($\because$ Voltage between any two terminals is line to line voltage or $V_L$)

Similarly, connecting between Y and B

$\begin{align*} \therefore \omega _2 &= …