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i in (-oo:+oo)
-15 = -i/6-21 // + -i/6-21
21-(-i/6)-15 = 0
(1/6)*i-15+21 = 0
1/6*i+6 = 0 // - 6
1/6*i = -6 // : 1/6
i = -6/1/6
i = -36
i = -36
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