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p in (-oo:+oo)
51 = (21*p)/5 // - (21*p)/5
51-((21*p)/5) = 0
(-21/5)*p+51 = 0
51-21/5*p = 0 // - 51
-21/5*p = -51 // : -21/5
p = -51/(-21/5)
p = 85/7
p = 85/7
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