Answer:
The axis of symmetry is x = 6
Step-by-step explanation:
To find this, first find the two x-intercept values and then take the average. This will always be the line of symmetry. 
x + 9 = 0
x = -9
x - 21 = 0
x = 21
Now take the average of these two numbers. 
(-9 + 21)/2
12/2
6
 
        
             
        
        
        
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