Yellow line in sketch

[jdaues]

Why is one line yellow?

My goal here is to create a 2-dimensional surface from which I can get the calculated area

[billbedford]

This line was projected from something you've deleted. 

[TrippyLighting]

The line is yellow because you projected it into the sketch from another sketch or the edge of a solid, or surface geometry, that was located earlier in the timeline. That isn't a recommended workflow!

Now the projection source is lost.

You can fix that: 

Here is a link to a blog post that describes the process.

[jdaues]

Where is that popup menu coming from?

[TrippyLighting]

Right-click on the sketch in the timeline.

If you have such questions, I recommend you go through one or more of the free tutorial courses Autodesk has created to help new users.

Those can also be accessed through the help menu in Fusion: