Your chemical equation is
"N"_2"O"_4 ⇌ "2NO"_2N2O4⇌2NO2
K_c = (["NO"_2]^2)/(["N"_2"O"_4])Kc=[NO2]2[N2O4]
We can setup an ICE table to solve this problem.
color(white)(mmmmmmml)"N"_2"O"_4 ⇌ "2NO"_2mmmmmmmlN2O4⇌2NO2
"I/mol·L"^"-1": color(white)(mll)0.0300color(white)(mmll)0I/mol⋅L-1:mll0.0300mmll0
"C/mol·L"^"-1":color(white)(mml) "-"xcolor(white)(mmm)"+2"xC/mol⋅L-1:mml-xmmm+2x
"E/mol·L"^-1": color(white)(m)0.0236color(white)(mml)2xE/mol⋅L−1:m0.0236mml2x
From the "N"_2"O"_4N2O4 concentrations, we see that
0.0300 - x = 0.02360.0300−x=0.0236
∴ x = "0.0300 - 0.0236 = 0.0064"x=0.0300 - 0.0236 = 0.0064
So, ["NO"_2]_text(eq) = 2xcolor(white)(l) "mol/L" = "(2 × 0.0064) mol/L" = "0.0128 mol/L"[NO2]eq=2xlmol/L=(2 × 0.0064) mol/L=0.0128 mol/L
K_"c" = 0.0128^2/0.0236 = "0.006 94"Kc=0.012820.0236=0.006 94