Hey alyosha2.
I can't see the image (broken link): can you please repost it or fix up the link?
Can't understand this.
The chain rule is used in third line but I can't understand that if Fx is f(x) then what is g(x)? I can't see any other function wrapping around Fx but then Fx just disapereas and we are let with just ((y-b)/a).