To find the price that maximizes Regina's revenue, we need to determine the vertex of the downward-opening parabola represented by the equation \( y = -0.5x^2 + 24x \). The x-coordinate of the vertex of a parabola given by the equation \( y = ax^2 + bx + c \) can be found using the formula:
\[ x = -\frac{b}{2a} \]
In this equation:
- \( a = -0.5 \)
- \( b = 24 \)
Substituting these values into the formula gives us:
\[ x = -\frac{24}{2 \times -0.5} = -\frac{24}{-1} = 24 \]
Therefore, the price at which Regina should sell her book to earn maximum revenue is $24.