Equilibrium help? I just need to understand how to do this!?

There are 2 X₂ molecules, 2 Y₂ molecules and 4XY molecules.
Mole ratio X₂ : Y₂ : XY = 2 : 2 : 4 = 1 : 1 : 2
X₂, Y₂ and XY occupy the same volume of the system.

Let c mol/L be [X₂].
Then, [Y₂] = c mol/L
and [XY] = 2c mol

Balanced equation for the reaction:
X2(g)+Y2(g)⇌2XY(g)

Equilibrium constant, Kc
= [XY]² / ([X₂] [Y₂])
= (2c)² / (c * c)
= 4c² / c²
= 4