Strayer Elementary Number Theory 1. 5. 77a.

I overlooked a small but very important part of the proof that should go at the beginning of each Claim after introducing the unknowns. I will just post it once right here, but note it.

"Now since ac-10bd=2, we clearly cannot have ac=0, otherwise 2=-10bd, which is impossible. It follows that a!=0 and c!=0."