it appears tha this will give you real roots.
in fact, it appears that one of the roots will be plus or minus c/a and the other root will be -1.
here's my worksheet that derives what i think is the answer.
apparently one of the roots will be -c/a and the other root will be -1.
i tested this out with various value of b = c + a to see if that was true.
as far as i can tell, it is.
i chose to test this against a quadratic solver.
i chose the following scenarios.
a b c root 1 root 2
-5 -25 -20 -4 -1
-5 +15 +20 -1 +4
+5 -15 -20 -1 +4
+5 +25 +20 -4 -1
in all of these scenarios, b = a + c.
one of the roots was -c/a and the other root was -1, as i had derived in the worksheet, confirming that the derivation was correct.
here's the results from the quadratic solver.
your solution is selection (a) real root