To factor this we need to use the reverse of the foil method. FOIL first, outside, inside, last. (A+B)(C+D) what times what (?+B)(?+D)will give you x^4? (x^2+b)(x^2+D). Next, what will give you -28 (A+?)(C+?) Several numbers will work but we want -4 and 7. This is why.. (x^2-4)(x^2+7) First times first gives you x^4 Outside times outside give you 7x^2 Inside times inside is -4x^2 and last times last is -28. combine 7x^2 and -4x^2 and you get your original middle number of 3x^2. now you know that (x^2-4)(x^2+7) is correct. Hope you learned something..
