Oppor it plenty no flow 
Mafo any body
        
             
        
        
        
Hey There! 
If i got this down right, 
An = A1 + (n - 1)d
Sn = A1 + A2 + A3 + ... + An
A1 is given by
Sn = n (A1 + An) / 2
For Example
An = A1 + (n - 1)d
= 6 + 3 (n - 1)
= 3 n + 3
n = 50
Hope This Helps!!!
        
                    
             
        
        
        
Answer:

Step-by-step explanation:
Let the unknown angle be x
As the interior angles in a triangle are added up to 180°

Hope this helps you.
Let me know if you have any other questions:-)