I'm doing the khan academy trying to learn calculus and I'm currently stuck in pre-algebra. Here is the problem: http://imgur.com/a/N0lEV Now basically I get to the part x^2 -7x+c = x^2 -2dx+d^2 The khan academy hint says, that by matching terms, c=d^2, now... I know how to solve this problem once I know that c=d^2 but how did the khan academy get that? EDIT: I've simplified it further. I'm looking at -7x+c=d(-2x+d), I still cant figure out how c=d^2 EDIT 2: and just for the heck of it, h...