We are given

R = -p³ + 33p² + 9p       (18 £ p £ 28)

To find the stationary points, we take the derivative, set it equal to zero, and solve for p:

R'(p) = -3p² + 66p + 9.
-3p² + 66p + 9 = 0   when
-p² + 22p + 3 = 0     Divide by 3

This is a quadratic equation whose solutions must be computed with the quadratic formula: p » $22.14 and R » $5,522.61.

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