A container initially has 0.0186 M ammonia at a certain temperature. When the system reaches equilibrium the concentration of ammonia is 0.0035 M. Calculate Kc for the following reaction as written.

Write the Kc expression from the equation..
N2 + H2 ==> 2NH3
Then 0.0186 M NH3 initially.
At equilibrium H2 must be 3x, N2 must be 2x, and NH3 must be 0.0186-2x. Plug those values in and solve for Kc.