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p in (-oo:+oo)
(1/3)*(6*p+21) = 15*(p-4) // - 15*(p-4)
(1/3)*(6*p+21)-(15*(p-4)) = 0
(1/3)*(6*p+21)-15*(p-4) = 0
1/3*(6*p+21)-15*(p-4) = 0
1/3*(6*p+21)-15*(p-4) = 0
67-13*p = 0
67-13*p = 0
67-13*p = 0 // - 67
-13*p = -67 // : -13
p = -67/(-13)
p = 67/13
p = 67/13
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