Chemistry whizzes come here

One of the reactions used to form sulfuric acid is the reaction of oxygen with
sulfur dioxide under equilibrium conditions to form sulfur trioxide.
Before the reaction, the concentration of sulfur dioxide was 0.06 mol L
and
the concentration of oxygen was 0.05 mol L
After equilibrium was reached, the concentration of sulfur trioxide was 0.04 mol L

Calculate the equilibrium constant, K, for the reaction. Show relevant working

Hellllp

Actually, you can represent the reaction that occurs in the atmosphere to make H2SO4(l) from SO2, oxygen and water like the reaction below. There is little to no measurable SO3 in the atmosphere.
2SO2(g) + O2(g) + 2H2O(g) --> 2H2SO4(l)

In one form of the industrial production of sulfuric acid, SO2 is produced and then oxidized to SO3 in the presence of oxygen and a catalyst, and the SO3 is reacted with water to make H2SO4.
....2SO2 ..+.. O2 --V2O5 catalyst--> 2SO3
I ..0.06M .... 0.05M ........................... 0
C -0.04M ... -0.02M ..................... +0.04M
E. 0.02M .... 0.03M ....................... 0.04M

Keq = [SO3]^2 / ([SO2]^2[O2])
Keq = 0.04^2 / 0.02^2 / 0.03
Keq = 133 ... to three significant digits

Technically, you can only express Keq to one significant digit because of the lack of precision in your concentrations. It is much more likely that the concentrations of the gases will be known to more precision than that.

============== Follow up ==============

Charlie has the right idea, but it worries me that he has so little respect for the science that he can't be troubled to use the "shift" key to make the proper symbols.

conc of so3 increases by 0.04
concentration of so2 decreases by 0.04 - molar ratio 1:1
concentration of 02 decreases by 0.02
use these changes to determine equilibrium concetraions and then put into formula for kc
hope i helped :)