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a in (-oo:+oo)
3/8-(7*a)-(6*a) = -829/8 // + -829/8
3/8-(7*a)-(6*a)-(-829/8) = 0
3/8-7*a-6*a+829/8 = 0
104-13*a = 0 // - 104
-13*a = -104 // : -13
a = -104/(-13)
a = 8
a = 8
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