Use the Keq equation.. concentraion of products divided by concentration of reactants at equilibrium. (with coefficients as exponents, none in this case reaction is balanced)
Keq = [PCl5] / ([PCl3] [Cl2])
fill in given values and solve for [Cl2] using algebra. Im just going to sub in "c" for the unknown.
0.00094 = 0.44 / 0.33c
0.0003102c = 0.44
c = 1418.44 mol/L