Joe, Firstly thank you for your time on the post. Definetly not board.
This is all new to me but I am trying to figure it out. Even just plugging variables can be difficult! I have followed you up to:

S = M ÷ Fb
bd²/6 = M/Fb
A = bd = (6 × M) ÷ (Fb × d)

Can you help me with the algebraic transition from the second line to the third?