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Thank you!

The second solution is working for me.
I put every item completely into \smash:

\begin{enumerate}[a)]
\item $\displaystyle \smash{z(x;y)=x-y}$
\item $\displaystyle \smash{z(x;y)=\frac{x}{y}}$
\item $\displaystyle \smash{z(x;y)=x+y}$
\end{enumerate}

Never used \smash before.

Kind ...

Hi all

in this example:


\documentclass[12pt]{article}
\pagestyle{empty}

\usepackage{multicol}
\usepackage[neverdecrease]{paralist}

\begin{document}
\begin{multicols}{3}
\begin{enumerate}[a)]
\item%
\setlength{\unitlength}{1cm}
\begin{picture}(0,0)
\linethickness{1pt}
\put(-1,0){\line(1,0){16 ...