How to show that line in 2D draw?

What do you mean by that line?

What are you trying to do?

Draw a pipe or just a flow chart of a pipeline?

You place a line by the coordinate system?

Or do you want to draw it in a certain length? 

Do you want to show or hide lines with the level selection? Create/assign a level with color and weight... turn on or turn off level... these are just two very simple ways to turn make a line; hide  or make it visible...

Of course I have not addressed  Poly lines or any other lines or setting which are needed and to be established in the seed-file, like Dimension lines, arrows....

In an AutoCad course; this will be the first thing you will learn 

thank you for your invitation

but the question is not clear 

I agree that question is also not clear to me.

this question is not clear to me.. can you please explain.