To calculate the gain due to Vin,cm to X and Y, we use equivalent circuit in figure (b).

Now, $I_{d1} = gm_1 ( V_{in,cm} - V_p )$ and $I_{d2} = gm_2 ( V_{in,cm} - V_p )$

That is,

$(g_{m1}+g_{m2})(V_{in.CM}-V_p)R_{SS}=V_p$

and

$V_p = \frac{(g_{m1}+g_{m2})R_{SS}}{(g_{m1}+g_{m2})R_{SS}+1}V_{in.CM}$

We now obtain output voltages as

$V_x= …