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a in (-oo:+oo)
a+7 = 21-6*a // - 21-6*a
a-(-6*a)-21+7 = 0
a+6*a-21+7 = 0
7*a-14 = 0 // + 14
7*a = 14 // : 7
a = 14/7
a = 2
a = 2
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