您可以使用以下flalign*
环境:
\documentclass{article}
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.3pt}
\usepackage{amsmath}
\begin{document}
\begin{flalign*}
& ax^2 + bx + c = 0 & & \boxed{x = \frac{-b\pm\sqrt{b^2-4ac}}{2a}}
\end{flalign*}
\end{document}