Factor By grouping.
4a^2b^2 - 9 (ab + c)^2
Must I FOIL (ab + c)^2 and distribute 9 before factoring?
No, $\displaystyle \ \ \ 4a^2b^2 - 9(ab + c)^2 $ is the difference of two squares.
$\displaystyle A^2 - B^2 \ = \ (A - B)(A + B)$
Determine what the square root of $\displaystyle \ \ 4a^2b^2 \ \ is \ \ and \ \ also \ \ what \ \ the \ \ square \ \ root \ \ of \ \ 9(ab + c)^2 \ \ is.$
Those will be your A and your B, respectively.