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a in (-oo:+oo)
(2/3)*a = -a-15 // + -a-15
(2/3)*a+a+15 = 0
5/3*a+15 = 0 // - 15
5/3*a = -15 // : 5/3
a = -15/5/3
a = -9
a = -9
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