We have the following equilibrium: 2 A(aq) ⇄ B(aq). At equilibrium we measure [A] = 0.056 M and [B] = 0.21 M. Calculate the equilibrium constant for the reaction as written.
2 A(aq) ⇄ B(aq).
K = (B)/(A)^2 = ?
Plug in the numbers at equilibrium and solve for K. I shall be happy to check your answer.