How can I create multiple lines of equations in flashcards?

Question

I have flashcards that work perfectly, but on a few of my flashcards I want to be able to give 2 equations on separate lines. Normally I know how to do this....such as using gather or align but when I try to use this, I'm getting errors that I don't understand. Below is my attempt, thanks for the help! (the flashcard package is something I downloaded, but I don't think it should matter)

\documentclass[avery5371,grid]{flashcards}

\usepackage[latin1]{inputenc}
\usepackage{amsfonts}
\usepackage{amsmath}

\cardfrontstyle[\large\slshape]{headings}
\cardbackstyle{empty}

\begin{document}

\begin{flashcard}[]{Lorentz transformation for motion along x (for t' and x')}
\vspace*{\stretch{1}}
\begin{displaymath}
\begin{align}
t' = \gamma \left(t - \frac{vx}{c^2} \\
x' = ....
\end{align}
\end{displaymath}
\vspace*{\stretch{1}}
\end{flashcard}

\end{document}

Of course this gives the error "! Package amsmath Error: Erroneous nesting of equation structures; (amsmath) trying to recover with `aligned'."