
has characteristic equation

which has roots at

. The negative root has multiplicity 2. So the general solution is
multiplication property of equality
To find the sum of the interior angles you use the formula (n-2)(180), where n is the number of sides. In this case, the number of sides is 12, so you get 1800 degrees. Then you divide by number of angles which is 12. So your answer is 150 degrees.
The judge to be probable shown is not a method