Answer:
The 21st term is 137.
Step-by-step explanation:
Arithmetic progression:
In an arithmetic progression the difference between consecutive terms is always the same, and its called common difference.
The nth term is given by:

In which  is the first term and d is the common difference.
 is the first term and d is the common difference.
The first two terms are -3, 4. 
This means that 
So


The 21st term is

The 21st term is 137.
 
        
             
        
        
        
Answer: 18
Step-by-step explanation:
Add them all up 
18+21+20+14+17=90
Divide by 5 since that is how many numbers there are 
90/5=18
 
        
                    
             
        
        
        
Answer:
Hexagon
Step-by-step explanation:
Answer: If an interior angle of a regular polygon measures 120°, it has 6 sides (Hexagon).