\documentclass[a4paper,11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[polish]{babel}
\usepackage{fourier}
\usepackage{amsmath}
\usepackage{geometry}

\geometry{width=126mm}


\begin{document}

\begin{equation}
\label{eq:function1}
f(x) = x^2 -7
\end{equation}

\begin{equation}
\label{eq:function2}
f(x) = x^3 -\frac{1}{2}x^2 + 1
\end{equation}

\begin{equation}
\label{eq:function3}
f(x) = \sqrt{x-1} + \frac{1}{3}x
\end{equation}

\begin{align}
\label{eq:function1n}
f(x) &= x^2 - 7 \\
\label{eq:function2n}
f(x) &= x^3 - \frac{1}{2}x^2 + 1 \\
\label{eq:function3n}
f(x) &= \sqrt{x - 1} + \frac{1}{3}x
\end{align}

\end{document}
