By self-scrutiny, what are the givens? ... If you can't solve the problem, go back and look at the givens? What are you taking for granted. 'Oh, I know that every time I see an x, that x equals 4.' Really? Maybe you wrote down 4, somehow, because you were thinking of 4 at the time, and x actually is a 3. So, just go back and look at what you're taking for granted.