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p in (-oo:+oo)
21/5 = 3*p // - 3*p
21/5-(3*p) = 0
21/5-3*p = 0
21/5-3*p = 0 // - 21/5
-3*p = -21/5 // : -3
p = -21/5/(-3)
p = 7/5
p = 7/5
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