$9a^2-a^4+2a^2b-b^2 $ how to factor this algebraic expression? Algebraic identities [closed]

Hint:

Separate the $9a^2$ out and focus on the remaining three terms:

$-(a^4-2a^2b+b^2)$

Allowing $u = a^2$ might make the algebraic identity (something to do with binomials) more visible. Move on from there.