In the equation N2 (g) + 3H2 (g)<---> 2 NH3 (g) the expression for the
equilibrium constant is:
a.  [NH3]2 / [N2][H2]3
b.  [2NH3]/[N2][3H2]
c.  [NH3]/[N2][H2]

Hi teatea!
The correct answer is 'a'. That is, the equilibrium constant is, by
definition, written as:

         2
    [NH3]
-------------
          3
  [N2][H2]

You can check the definition of equilibrium constant given a balanced
reaction in the following link.

SCIMEDIA: The Equilibrium Constant Expression
http://elchem.kaist.ac.kr/vt/chem-ed/courses/equil/intro/eqconst.htm


Google search strategy
equilibrium constant
://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=equilibrium+constant


I hope this helps! If you have any doubt regarding this answer, please
don't hesitate to request a clarification.


Best wishes!
elmarto