Question 598042
<pre>
Sum of the n interior angles = 180°(n-2) 

Sum of all but 1 of the n interior angles = 156°(n-1)

Sum of all n interior angles = 156°(n-1) + 84° 
  
So

180°(n-2) = 156°(n-1) + 84°

Solve that and get 12 sides

Edwin</pre>