Given the following reaction at equilibrium, equilibrium concentrations and value of K, determine the unknonw concentration:

C₃H₈ (g) + 5O₂ (g) ➞ 3CO₂ (g) + 4H₂O (l)

where k = 15 and [CO₂] = 1.10 X 10^-4M, [H₂O] = 3.95 X 10^-5M, [C₃H₈] = 4.21 M, and [O₂] = ? M