Calculate Kc for 4NOBr decomposition from given equilibrium data

[aaronfue]

Need help with equilibrium equations -- ASAP Please

Homework Statement

I'm studying for a final that I'm taking tomorrow and am stuck on the following:

Calculate K_c for the reaction:

4NOBr (g) $\Leftrightarrow$ 4NO (g) + 2Br₂ (g)

given the following information:

2NO(g) + Br₂ (g) $\Leftrightarrow$ 2NOBr (g); K = 2.0

Homework Equations

Not sure about which equations other than K_c = $\frac{[products]}{[reactions]}$

The Attempt at a Solution

I can't even start this answer. My first assumption of where to start was to write the second equation backwards, just so it looked like the first!??

 

[epenguin]

I rarely remember the conventions about which way up stuff is expressed, as in anything you ever read the constants will usually be defined. However it didn't take a minute to look this up e.g. http://www.chemguide.co.uk/physical/equilibria/kc.html

 

[janhaa]

$$K_c=(1/K)^2$$