Given the balanced equation 2 H2 + 1 O2 <-> 2 H2O

The equilibrium constant for the reaction is 9.4 x 10^13. If the equilibrium concentration if water vapor is 1.82M, what is the concentration of water?

Please help as soon as possible. I am so lost, please explain with detail.

Simply use the equilibrium expression.

K_c = \frac{[H_2O]^2}{[H_2]^2[O_2]}

Let x be a certain concentration, that of oxygen.
The concentration of H2 will be 2x
and that of O2 will be x

Solve for x.

and one last trivial thing, when you have only one compound/element, don't put 1, it is implied to be 1 when you omit it.