Your error is that "a" is not 0, it's -1, because if you square the
parentheses and collect terms, you get









The - sign changes all the signs inside the parentheses:







So you see "a" is not 0, it's -1.

I multiplied it all out to show you that a =-1, not 0. But you 
can tell it is -1 without multiplying it out if you think about it.

Edwin