Answer:
x = 7
or
7 + 10 + 4 = 7 + 10 + 4
Explanation:
( ? + 10 ) + 4 = 7 + ( 10 + 4 )
x + 10 + 4 = 7 + 10 + 4 (the answer is just 7)
x + 14 = 21
x = 21 - 14
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