My teacher posted an answer sheet in preparation for a test coming up about factoring polynomials. One of the questions was... Completely factor the expression y^4-255. Step 1. (Y^2+16)(y^2-16). Step 2. (Y+4)(y-4)(y^2+16). I'm having trouble understanding why she didn't factor (y^2+16) in step 2 down further. I'm also having trouble with the positive and negative signs in this problem (when to use positive vs negative)