Question 168695
 Factor....Need some help 
x^4-13x^2+36
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It will be (x^2 + ?)*(x^2 + ?)
The ?'s product will be 36, and they add up to -13.
Since it's +36, they have to have the same sign, both Pos or both Neg.
Since they add to -13, they'll have to be both negative.
Consider the factors of 36:
1*36
2*18
3*12
4*9, and 
6*6
They're the same sign, so that add to 13.  If the were not the same sign, you would look at the differences.
Only 4 and 9 add to 13, so they're it.
It's (x^2 - 4)*(x^2 - 9)