% eqbox.sty
NeedsTeXFormat{LaTeX2e}[1994/06/01]
ProvidesPackage{eqbox}[2019/01/01 Boxed Equations]

RequirePackage{xcolor}
RequirePackage{pgfopts}
RequirePackage{amsmath}
RequirePackage{fancybox}
RequirePackage[most]{tcolorbox}

pgfkeys{
    /eqbox/.cd,
    colframe/.store in = colframe,
    colframe = black,
    colback/.store in = colback,
    colback = white,
    shadow/.store in = shadow,
    shadow = undefined,
}
ProcessPgfPackageOptions{/eqbox}

tcbset{
    highlight math style={
        enhanced,
        sharp corners,
        breakable,
        colframe=colframe,
        colback=colback,
ifxundefinedshadow
else
        shadow={2pt}{-2pt}{0mm}{shadow},
fi
        boxrule=0.4pt,
        boxsep = 3pt,
        left = 0pt,
        right = 0pt,
        top = 0pt,
        bottom = 0pt
    }
}

documentclass[letterpaper, 11pt, onecolumn]{article}

usepackage{lipsum}
usepackage{eqbox}

begin{document}
lipsum[1]
begin{equation}
tcbhighmath{x^2 + 3}
end{equation}
lipsum[1]
end{document}

ifdefinedshadow
        shadow={2pt}{-2pt}{0mm}{shadow},
fi