9+10ab-a^2-25b^2
Are you supposed to factor this?
$\displaystyle 9 + (-a^2 + 10ab - 25b^2)$
$\displaystyle = 9 - (a^2 -10ab + 25b^2)$
$\displaystyle = 9 - (a - 5b)^2$
$\displaystyle = 3^2 - (a - 5b)^2$ <-- This is the difference of two perfect squares
$\displaystyle = [ 3 + (a - 5b) ] [ 3 - (a - 5b) ] $
$\displaystyle = (3 + a - 5b)(3 - a + 5b)$
-Dan