Dynamic Block Stretch Help, triangle in center of rectangle while stretching

[Anonymous]

I have this awning window in front elevation and i am trying to make it into a dynamic block by stretching its width and hieght but i need the trangle it remain at the center. is this possible can someone help me ? 

I provided and Attachment with it 

[Libbya · Accepted solution]

Here's a screencast that shows the step-by-step procedurefor both of your window blocks.