Factor this expression completely 9a4 - b2

First write out all of the factors you have for each term (I am also making the assumption that the expression is 9a^4 - b^2, where ^ is signifying an exponent)
(3*3*a*a*a*a) - (b*b*1)
Seeing as how there is nothing to factor out of either term, your answer is:
(3a^2 + b)(3a^2 - b) you can check that this is correct by distributing this:
(3a^2)(3a^2 - b) and (b)(3a^2 - b). Let me know if this doesn't make sense!