Typo: "Now, the are given..." To be read: "Now, they are given..." Ritthe, there is no square of negative number in Galaab's question. He just expanded -6 to 4 - 10 and 9 - 15. And, you can manipulate those numbers to be in (a-b)^2 = a^2 + 2.a.b + b^2.