Fill in the blank with a constant, so that the resulting expression can be factored as the product of two linear expressions: 2ab - 12a + 3a + 3b - 8b - ab + ___
Simplify as
ab - 9a - 5b
Take the product of the two coefficients on a, b = (-9) (-5) = 45
(This is the number that goes in the blank)
Add this
ab - 9a - 5b + 45 factor as
(a - 5) ( b - 9)
+2653 
