Question 256125
We are given
(i) {{{x^4-8x^2-9}}}
step 1 - set up a dummy variable like U = x^2. If U = x^2, then U^2 = x^4. 
Step 2 - substitute the dummy variable into the equation to get
(ii) {{{U^2 -8U -9}}}
step 3 - factoring we get
(iii) {{{(U-9)(U+1)}}}
step 4 - replace U with x^2 to get
(iv){{{(x^2-9)(x^2+1)}}}
step 5 - notice the difference of squares. We can factor that to get
(v) {{{(x-3)(x+3)(x^2+1)}}}