Calculate the concentration of the products of the following equilibrium

NOBr2 (g) ----> NO (g) + Br2 (g)
Equilibrium constant = 12.3
Equilibrium concentration of NOBr2 = 2.17 mol/L

Assuming that you began with just NOBr2, then at equilibrium, [NO] = [Br2] = X

Then, K = 12.3 = [NO][Br2]/[NOBr2] = X^2 / 2.17
X = 5.17 M = [NO] = [Br2]