The answer is (y-7x)(y+7x). If you writ it out, y x y is y^2, y(7x) is 7xy, y(-7x) is -7xy, (7x)(-7x) is -49x^2. If you write them into a factor form, it would be y^2 +7xy -7xy - 49x^2. But -7xy and 7xy would cancel out because it equals to zero. So that leaves to - 49 x squared subtracted from y squared. And that’s how you got the answer above.
