I'm doing some math over the summer for fun. I have a very weak background in math, but have come around and thoroughly enjoy it. Can someone explain what is going on with this problem involving Newton's Quotient?

I was doing fine, until I hit some a algebra concept I'm unfamiliar with. It's the third photo were my pencil is pointing to. The numerator suddenly become 15(2a-3)-15(2a+2h-3). I don't understand how that is.