I am trying to figure out the angels for a pyramid triangle where the 4 pyramid sides are 18" wide on the bottom, 21" from the tip to the middle of the bottom (height) by .75" thick. The base is an 18" square by .75" thick. I am attaching a picture of what I am trying to make. Can you tell me the angels that I need to cut for both the sides and bottoms of the pieces? Or at least how to figure it out? I have been going crazy, as it seems as though it should not be that difficult, and it's been YEARS since I had geometry!!! Thank you for any help you might be able to provide.