-10-8a=8-10a
+10 +10
-8a = 18-10a
+10a +10a
2a = 18
2 2 
a = 9 
You’re supposed to divide the 2 at the end. Hopefully this helped:)
        
             
        
        
        
Answer:
x = 91
Step-by-step explanation:
The opposite angles of an inscribed quadrilateral are supplementary, that is
∠ A + ∠ C = 180, thus
x + 89 = 180 ( subtract 89 from both sides )
x = 91
 
        
             
        
        
        
-4x +12 -12 = -4 -12
-4x = -16
x = 4