Answer:
The answer is 120 degrees.
Step-by-step explanation:
 
        
             
        
        
        
A rectangle has 2 diagonals
        
             
        
        
        
Answer:
a.  
b. x = 4 (see steps below)
Step-by-step explanation:
Since the two polygons are similar, the sides are proportional to each other.  Using a proportion, you can solve for 'x' using cross-multiplication and division: 

Cross-multiply: 15(x + 3) = 21(x + 1) 
Distribute:  15x + 45 = 21x + 21 
Subtract '15x' from both sides: 15x + 45 - 15x = 21x + 21 - 15x or 45 = 6x + 21
Subtract '21' from both sides: 45 - 21 = 6x + 21 - 21 or 24 = 6x 
Divide by 6 on both sides: 24/6 = 6x/6 
Solve for x: x = 4