Question 839558
You should at least try the different combinations which might give 8 for the coefficient on a^2, and -5 for the coefficient on b^2.  Test each arrangement to find the combination or arrangement which works.


(1___   )(8___   )
(2___    )(4___    )
-
and for each of those, you have
(____-1)(___5)
(____1)(____-5)
(____-5)(____1)
(____5)(____-1)


You know you will have up to eight different arrangements to check.