Let's set up an ICE table to solve the problem.
$color(white)(mmmmmmmll)"N"_2"O"_4 ⇌ "2NO"_2$
$"I/mol·L"^"-1":color(white)(mm)0.0500color(white)(mmll)0$
$"C/mol·L"^"-1":color(white)(mml)"-2"xcolor(white)(mml)"+2"x $
$"E/mol·L"^"-1":color(white)(l)0.0500 - xcolor(white)(mll)2x $
$K_c = (["NO"_2]^2)/(["N"_2"O"_4]) = (2x)^2/("0.0500-"x) = 0.513$
$(4x^2)/("0.0500-"x) = 0.513$
$4x^2 = 0.513("0.0500-"x) = "0.025 65" - 0.513x$
$4x^2 + 0.513x - "0.025 65" = 0$
$x = "0.038 46"$
∴ $["N"_2"O"_4] = ("0.0500-"x) color(white)(l)"mol/L" = "(0.0500 - 0.038 46)"color(white)(l) "mol/L" = "0.0115 mol/L"$
$["NO"_2] = 2x color(white)(l)"mol/L" = "2 × 0.038 46 mol/L" = "0.0769 mol/L"$
Check:
$K_c = (["NO"_2]^2)/(["N"_2"O"_4]) = 0.0769^2/0.0115 = 0.514$
Close enough! It checks!