**Consider the following balanced chemical equation:**

CH4(g) + H2O(g) ⟷ CO(g) + 3 H2(g)

A reaction mixture initially contains 3.1 M CH4 and 1.90 M H2O. Determine the equilibrium concentration of CH4 if Kc for the reaction at this temperature is 2.10 x 10-5.

I know how to start this problem, just having trouble finishing it.

When solving after using the quadratic formula I get 3.1 = 27x^4 and 1.9 = 27x^4; however, after solving for x neither of these are correct. The final right answer is x = 0.0463 and the equilibrium concentration of CH4 is 3.0537. Does anybody have an idea of where I'm going wrong here?