Hello, I am reviewing some algebra concepts for a class I will be taking in the spring. I a trying to factor the polynomial: x4-11x2y2+y4. I know the answer is (x2-3xy-y2)(x2+3xy-y2). A hint was given to add a term to make a complete square, and then immediately subtract the term which yields a difference of two squares which can then be factored. I cannot figure out what term to add to this equation to put it in the form of a difference of two squares. If someone could explain to me the steps to take in order to factor this, or provide some intuition on how this works I would greatly appreciate it. Thank you for your time and consideration.