I write a textbook and I have completed 40 pages, but I don't like that the texts inside the $..$ are italic. For example
(x-3y) not like
(x-3y) I want
\documentclass{article}
\begin{document}
$(x-3y)$
\end{document}
What can I do to make the texts non-italic? Also, I don't want to change them all one by one.