
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{mathtools}
%\usepackage{amssymb}
\pagestyle{empty}
\begin{document}
\[
\forall \lambda \in \mathbb{R} 
\quad
\int _{a}^{b}\lambda \, f(x)
\, \mathrm{d} x 
= 
\lambda \, \int _{a}^{b}f(x)
\, \mathrm{d} x
\]
\end{document}
