It is true in every equilibrium calculation that the equilibrium concentration is equal to the initial concentration plus the change caused by the reaction.  Since we generally expect the concentration of a reactant to decrease, it is logical to make the change in a reactant negative.  Likewise, we expect the concentration of a product to increase, so the the change in a product is positive.

So, for an imaginary reaction:
 

	A	+ 2 B		C	+ 3 D
Initial	[A]	[B]		[C]	[D]
Reaction change	-rxn	-2rxn		+rxn	+3rxn
Equilibrium	[A]-rxn	[B]-2rxn		[C]+rxn	[D]+3rxn

where:
[A], [B], ... represent the initial concentrations of substances in the reaction
rxn represents the change in concentration caused by the reaction, and is the only quantity that depends directly on the reaction coefficients

You can use this chart to fill in any of the values that are missing.  The Equilibrium row is always a key in this table.  The values in this row can be used to calculate the numerical value of K_eq.  Or, if K_eq is known, then the values in the Equilibrium row can be calculated from the K equation, and used to complete other missing values in the table.

Download the following two Excel spreadsheets.  Then load them into Excel and work through the examples and questions on each.
Equilibrium Spreadsheet 1
Equilibrium Spreadsheet 2