We obviously have to multiply the numerator and the denominator by the same thing so that we actually don't change the value of the expression. If this right over here had a plus 3 then we would do a minus 3 here. This is a technique that we learn in algebra or sometimes in pre-calculus class to rationalize usually denominators but to rationalize numerators or denominators. It's also a very similar technique that we use oftentimes to get rid of complex numbers usually in denominators. But if you multiply this out, and I encourage you to do it, you notice this has the pattern that you learned in algebra class. This is the difference of squares. Something minus something times something plus something.