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f in (-oo:+oo)
(2/3)*f-8 = -15 // + 15
(2/3)*f-8+15 = 0
2/3*f+7 = 0 // - 7
2/3*f = -7 // : 2/3
f = -7/2/3
f = -21/2
f = -21/2
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