Tikz for parallelogram problem

[karush]

https://mathhelpboards.com/threads/3-coordinates-of-parallelogram-stuv.6195/

this problem had over 6000 views so I was interest in posting it to Linkedin hoping it would recruit some new members

however it had a graph in the OP which has been deleted but another was posted using some code shown here

https://www.physicsforums.com/attachments/312510._xfImport

I am trying to redo this graph for post to Linkedin but not sure if I understand it
what is the (5,15) and the -9s
apparently this is an attempt to draw the parallelgram

nor sure why my tikz did not render probably don't need the dots just lines
anyway be nice to post something impressive on Linkedin of which I have received curiosity

my tikz code...
\begin{tikzpicture}[xscale=.5,yscale=.5]
\draw [thin] (-7,0) -- (8,0);
\draw [thin] (0,-2) -- (0,16);
\draw[thin][fill] (-2,-2) circle (.1);
\draw[thin][fill] (-1,-1) circle (.1);
\draw[thin][fill] (0,0) circle (.1);
\draw[thin][fill] (1,1) circle (.1);
\draw[thin][fill] (2,2) circle (.1);
\draw[thin][fill] (3,3) circle (.1);
\draw[thin][fill] (4,4) circle (.1);
\draw[thin][fill] (5,5) circle (.1);
\draw[thin][fill] (6,6) circle (.1);
\draw[thin][fill] (7,7) circle (.1);
\node

at (7.1,7) {T(7,7)};
\draw[thin][fill] (6,8) circle (.1);
\draw[thin][fill] (5,9) circle (.1);
\draw[thin][fill] (4,10) circle (.1);
\end{tikzpicture}​

 

[NateD]

The (5,15) and the -9s refer to the coordinates of the points that make up the parallelogram. The first coordinate, 5, is the x-coordinate of one corner of the parallelogram and the 15 is the y-coordinate. The -9s refer to the x-coordinates of the other three corners of the parallelogram.