Answer:
b = 18 units
Step-by-step explanation:
 
        
             
        
        
        
Step-by-step explanation:
✧ 
- Set up an equation and solve for x :
⋆  [ Exterior angle is always equal to the sum of two non - adjacent interior angles ]
 [ Exterior angle is always equal to the sum of two non - adjacent interior angles ]
~Remove the unnecessary brackets
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~Add the like terms : 2x ° & 3x°
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~Move 4x to right hand side and change it's sign
~Similarly move 12 to left hand side and change it's sign
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- Now , Substituting the value of x in (4x+9)° [ Exterior angle ]
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