When editing the following document with Overleaf
\documentclass[a4paper, 12pt,fleqn]{article}
\usepackage{amsmath}
\usepackage{breqn}
\usepackage[utf8]{inputenc}
\usepackage{indentfirst}
\title{Exercice sur les équations}
\begin{document}
\maketitle
\begin{align*}
12 + x &= 17 \\
12 \boxed{- 12} + x &= 17 \boxed{- 12} \\
x &= 5
\end{align*}
\begin{align*}
3x – 2 &= 17 \\
3x - 2 \boxed{+ 2} &= 17 \boxed{+ 2} \\
3x &= 19
\end{align*}
\end{document}
I get the expected output
but there is a warning (at least I think this is a warning due to the yellow triangle)
Is this something serious?
(I apologize if this is obvious, I am back to LaTeX after 20 years trying to help my children with the formatting of their homework after seeing with disgust what they were about to send to the teacher (we are in France, confined at home))
–
(U+2013) for the minus sign, but a simple hyphen. – egreg Mar 28 '20 at 18:263x – 2 &= 17
change this to3x - 2 &= 17
– Ulrike Fischer Mar 28 '20 at 18:27