9a4-b2=36a-2b=2(18-b) But if you meant 9a^4-b^2 it is not possible to factor
You can start by using the formula for the difference of two squares. Actually, after that I don't think you can factor it any further.
a2-b2 = (a-b)(a+b)
(a + b)(a - b)
b2(9b-8)
It cannot factor without imaginary numbers
The GCF is a2b.
a2-b2=(a-b)(a+b)
(3a + b)(3a - b)
4(16a4 + b2)
(b + 2)(b - 8)
b(b - 8)