\usepackage[fleqn]{amsmath} is left-aligned by default and is set globally, so after that for in-paragraph formulas even if you add \centering or \begin{center}... \end{center}can't do anything to center the formula.
Solution: Remove[fleqn], read\usepackage{amsmath}, the latter formula does not need to add any additional\centering or \begin{center}... \end{center}command, you can achieve the default center, just all the formulas are no longer globally aligned, as long as the layout requirements of the formula all centered on the line.
As an added bonus, I wrote the equation using \begin{equation}. \end{equation}