How to tell how far equilibrium is?
Try writing out the K expression. Remember that in general, 

${K = \frac{[product]}{[reactant]}}$

In your case, ${K = \frac{[C]^2}{[A][B]}}$ 

Or in other words, 

$K = \frac{[product]^2}{[reacant][reactant]} = \frac{[product]^2}{[reacant]^2}$  

I combined the $[reactant]$ terms because the equation you have, there is 1:1 ratio of both reactants. 

We can take the square root of both sides: 

$\sqrt{K} = \sqrt\frac{[product]^2}{[reactant]^2} = \frac{[product]}{[reactant]}$ 

So now just plug in your numbers to determine how far to the right the equilibrium lies. 

$\sqrt{K} = \sqrt{4.2} = \frac{[product]}{[reactant]} = 2.0$ 

Therefore (upon consideration of reaction stoichiometry), there is a 2:1:1 ratio of products to reactants at equilibrium. 

We can also plug in the Q (reaction quotient) value; remember that K is just a special Q value (K is the Q value when the system is at equilibrium):

$\sqrt{Q} = \sqrt{1.0} = \frac{[product]}{[reactant]} = 1.0$ 

The Q value indicates that the system currently lies away from the equilibrium mix of a 2:1:1 ratio between products and reactant gases. At Q = 1.0 we have a 1:1:1 mix of A, B, and C.