Thanks, heureka....here's one other way
Let the polynomial be a(x + 1)(x + 2)(x - q)
Where q is the unknown root
Since the lead coefficient is 1, a = 1
So. expanding this, we have
(x^2 + 3x + 2)(x - q ) = x^3 + 3x^2 + 2x - qx^2 - 3qx - 2q
Equating coefficients, we have
(3 - q) = -a
(2 -3q) = -b
-2q = -8 ⇒ q = 4 = the unknown root
So
(3 - 4) = -a ⇒ -1 = -a ⇒ 1 = a
( 2 - 3(2) ) = -b ⇒ -10 = -b ⇒ 10 = b
