The first step in any equilibrium problem like this is to write the complete balanced equation:
$NH_4Cl(s) harr NH_3(g) + HCl(g)$
The form of $K_c$ for this reaction (the subscript c means concentration, and the standard concentration is in units of $"mol/L"$) is:
$K_c = [NH_3][HCl]$
The $NH_4Cl$ does not appear in the equilibrium expression because it is a solid and has a thermodynamic activity of 1.
The starting amount of reactant is $(2.83g)/(59.49 g/(mol))=0.0476 mol$
Because 40% of the reactant decomposed, the equilibrium concentrations are:
$[NH_3]_e = [HCl]_e = (0.0476mol times 0.40)/(2L)=0.00951 M$
The value of $K_c$ is therefore $K_c=(0.00951)^2=9.05 times 10^-5$