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a in (-oo:+oo)
7-(6*a) = 7*a-123 // - 7*a-123
7-(6*a)-(7*a)+123 = 0
7-6*a-7*a+123 = 0
130-13*a = 0 // - 130
-13*a = -130 // : -13
a = -130/(-13)
a = 10
a = 10
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