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Note: In sketching mode, there is Undo features that you can use if you make any mistake.

https://confluence.cornell.edu/download/attachments/112041403/Select+Sketching+Toolboxes.gif

 
On the right, there is a Graphic window. At the lower right hand corner of the Graphic window, click on the +Z axis to have a normal look of the XY Plane.

https://confluence.cornell.edu/download/attachments/112041403/Select+Normal+View.gif

In the Sketching Toolboxes, select Line. In the Graphics window, create three rough lines from starting from the origin in the positive XY direction (Make sure that you see a letter P at the origin and at each connection between the lines. The letter P the geometry is constrained at the point.)
You should have something like this:

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