Help Joining Hexagons with Triangles

Attempting a hexagon quilt featured on Sewing with Nancy. Hexagons were joined together by triangles to make strips, then strips were sewn in rows - no set-ins, beautiful and I thought I could do it until I started to cut the triangles (HORRORS). I've got 88 (8 1/2") black and white hexagons cut out Each of the six corners are 3 1/2" For the life of me I can't figure out the size of the triangles used to join the hexys . I thought the bottom of the triangle should be 3 1/2" (maybe a bit more from trimming), but I can't figure how tall the triangle should be.

Does anyone have a clue to what I'm trying to say, has someone seen this show, familiar with this method, or know some secret math calculation to make this work.

I'm so frustrated, I'm about ready to throw 88 hexys out the window. Sadly, I can remember a time when I would have eyeballed this project and had it done in a few days. Doctors and sons keep encouraging me to not give up because these types of tasks challenge my brain and is good for me - it's getting harder and I've got soooooo...... many more quilts to make.

Sorry to ramble, Thanks all, prayers are appreciated.