Red strikethrough line behind text

\documentclass{article}
\usepackage{xcolor}
\makeatletter
\newcommand{\linethrough}{\mathpalette\@thickbar}
\newcommand{\@thickbar}[2]{%
    \textcolor{red}{
    {\mkern0mu\vbox{
    \sbox\z@{$#1\textcolor{black}{#2}\mkern-1.5mu$}%
    \dimen@=\dimexpr\ht\tw@-\ht\z@+2\p@\relax % The +2 represents the vertical shift of the line.
    \hrule\@height0.5\p@ % The 0.5 represent the thickness on the line.
    \vskip\dimen@
    \box\z@}}#1}
}
\makeatother
\newcommand{\mathstrike}[1]{\ensuremath{\linethrough{#1}}}

\begin{document}
    \mathstrike{f(x) = x^2},\qquad \mathstrike{c_0}, \qquad \mathstrike{f(x) = x^2 + c_0}
\end{document}