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a in (-oo:+oo)
1/2-(3*a) = -71/2 // + -71/2
1/2-(3*a)-(-71/2) = 0
1/2-3*a+71/2 = 0
36-3*a = 0 // - 36
-3*a = -36 // : -3
a = -36/(-3)
a = 12
a = 12
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