You could just define a pic that draws a parallelogram. A parallelogram is defined by the lengths of two adjacent sides and the angle between them. These are store in the keys x, y and angle, respectively, in the parallelogram directory. With name prefix you can make the coordinate names unique if you draw several of them. To draw a parallelogram you only need to say
\draw (<x>,<y>) pic[name prefix=para1-]{parallelogram={x=<lenght 1>,y=<lenght 2>,angle=<angle>}};
Here is an example (if we drop (<x>,<y>) it is the default coordinate (0,0)).
\documentclass[tikz,border=7mm]{standalone}
\begin{document}
\begin{tikzpicture}[pics/parallelogram/.style={code={
\tikzset{parallelogram/.cd,#1}
\draw[pic actions] (0,0) coordinate (A)
-- ++ (0:\pgfkeysvalueof{/tikz/parallelogram/x}) coordinate (B)
-- ++ (\pgfkeysvalueof{/tikz/parallelogram/angle}:\pgfkeysvalueof{/tikz/parallelogram/y}) coordinate (C)
-- ++ (180:\pgfkeysvalueof{/tikz/parallelogram/x}) coordinate (D)
-- cycle;
}},parallelogram/.cd,x/.initial=1,y/.initial=1,angle/.initial=45]
\draw pic[name prefix=para1-]{parallelogram={x=4,y=2.5,angle=45}};
\draw (para1-A) -- (para1-C) node[pos=0.33,sloped,above]{$u+y$};
\draw (para1-D) node[above left]{$u$} -- (para1-B) node[below right] {$y$}
node[pos=0.33,sloped,above]{$u-y$};
\end{tikzpicture}
\end{document}

\usetikzlibrary{calc}and try,\path ($(A)!0.5!(C)$)--(B)node[midway,below,left=1mm]{$x-y$}; \path ($(A)!0.5!(C)$)--(C)node[midway,below]{$x+y$};– Aug 27 '19 at 10:24xslant=1,xscale=3since they are changing bases vectors. – Sigur Aug 27 '19 at 10:58