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a in (-oo:+oo)
7-a = 3*a+15 // - 3*a+15
7-a-(3*a)-15 = 0
7-a-3*a-15 = 0
-4*a-8 = 0 // + 8
-4*a = 8 // : -4
a = 8/(-4)
a = -2
a = -2
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