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