(a - b)^2 x^2 - 4abx - (a-b)^2
solve for x by factorisation method.
The problem is an old one. You have not copied it correctly. It should be
then it can be factored.
In mathematics, you don't understand things. You just get used to them.
I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.