To determine the price at which Regina should sell her book to maximize revenue, we can analyze the given revenue equation:
\[ y = -0.5x^2 + 24x \]
This is a quadratic equation of the form \( y = ax^2 + bx + c \), where \( a = -0.5 \) and \( b = 24 \). The maximum revenue occurs at the vertex of the parabola, which opens downwards since \( a < 0 \).
The x-coordinate of the vertex can be found using the formula:
\[ x = -\frac{b}{2a} \]
Substituting the values of \( a \) and \( b \):
\[ x = -\frac{24}{2(-0.5)} \] \[ x = -\frac{24}{-1} \] \[ x = 24 \]
So, the price Regina should sell her book to earn maximum revenue is:
$24