A △  Has to have 180º so 72+56 b would have to be appx. 52degrres 
        
             
        
        
        
Answer:
11 book shelves 
Step-by-step explanation:
All you have to do is divide 69/9 =10.66 
round up and get 11 book shelves
 
        
             
        
        
        
Answer:
Figure D is regular polygon.
 
        
             
        
        
        
Answer:
15. 122.49  16.13.17
Step-by-step explanation:
 
        
             
        
        
        
You are changing the sign on one so to keep an equilibrium you change the other sign too