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z in (-oo:+oo)
12 = 15*(z-(1/5)) // - 15*(z-(1/5))
12-(15*(z-(1/5))) = 0
12-15*(z-1/5) = 0
12-15*(z-1/5) = 0
15-15*z = 0
15-15*z = 0
15-15*z = 0 // - 15
-15*z = -15 // : -15
z = -15/(-15)
z = 1
z = 1
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