2NH3 ::equil::N2+3H2
Ke = [N2][H2]^3 / [NH3]^2
Take the [NH3]^2 to the other side to give:
Ke * [NH3]^2 = [N2][H2]^3
Now take the [H2]^3 to the other side to give:
Ke * [NH3]^2 / [H2]^3 = [N2]
Now, substitute in the known values:
0.612 * [0.731]^2 / [0.523]^3 = [N2]
Therefore [N2] = 2.29
Assuming the numbers you supplied are correct, this is how the problem is solved