所以我的概念是：
\documentclass[avery5388,grid,frame]{flashcards}
\usepackage{amsmath}
\usepackage{graphicx}
\cardfrontstyle[\large\slshape]{headings}
\cardbackstyle{empty}
\begin{document}
\cardfrontfoot{Calculus}
\begin{flashcard}[\raisebox{\depth}{\scalebox{-1}[-1]{Calculus}}]{Derivation}
\medskip
\textbf{Formula:}
\begin{align*}
P(x) &= x^2+p\,x+q \\ %this & is causing the problem I assume.
&= ...
\end{align*}
\end{flashcard}
\end{document}