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a in (-oo:+oo)
3/7 = a/12 // - a/12
3/7-(a/12) = 0
3/7-1/12*a = 0 // - 3/7
-1/12*a = -3/7 // : -1/12
a = -3/7/(-1/12)
a = 36/7
a = 36/7
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