How can I cut out a shape, but only three edges?

Hello!

I am trying to figure out how to cut out a shape, in a piece of Material that is “precut”

I have created this picture to explain what , and why I am trying to do it.
I cant think of a way to do this, or even describe it, so if anyone has a suggestion, it would be appreciated.

Referencing the picture!

1. I have a piece of acrylic that is the Shape of the GREEN outline in the picture.
2. I am trying to CUT OUT the lines that are BLACK in the picture
3. however, I would like too NOT cut out the black lines below the RED line.
What I would like to do, Cut all the way till it gets to Intersection A, and then start cutting again when it gets to intersection B!