Answer
Please see the work below.
Work Step by Step
We know that
$K_c=1.23\times 10^3=\frac{[COCl_2]}{[CO][Cl_2]}$
$[COCl_2]=(1.23\times 10^{23})[CO][Cl_2]$
We plug in the known values to obtain:
$[COCl_2]=1.23\times 10^{23}(0.012)(0.025)=0.369M$