How do I make this expression in one line?

Question

How do I make this expression in one line ?

$ (\forall (a ; b ) \in \mathbb{R}^{2})
(\forall (x ; y) \in \mathbb{R}^{2})
;
(a ; b )\star(x ; y) = \left(\dfrac{ax + by}{2};\dfrac{ay + bx}{2}\right)
$ 

Answer 1

(too long for a comment, hence posted as an answer)

There seems to be no problem displaying the formula on a single line when using the article document class, Computer Modern text and math fonts, and a 12pt basic font size.

The vertical lines at the edges of the screenshot are drawn because the showframe package is loaded.

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}

\usepackage{showframe} % display framelines around text block

\begin{document}
\[
(\forall (a ; b ) \in \mathbb{R}^{2})
(\forall (x ; y ) \in \mathbb{R}^{2});
(a ; b )\star(x ; y) = 
\biggl(\frac{ax + by}{2};\frac{ay + bx}{2}\biggr)
\]
\end{document}