To find the price that maximizes Regina's revenue, we need to determine the vertex of the quadratic equation given by \( y = -0.5x^2 + 24x \).
For a quadratic function in the form \( y = ax^2 + bx + c \), the x-coordinate of the vertex (which gives the maximum revenue in this case since the parabola opens downward) can be found using the formula:
\[ x = -\frac{b}{2a} \]
Here, \( a = -0.5 \) and \( b = 24 \).
Substituting these values into the formula:
\[ x = -\frac{24}{2 \cdot -0.5} = -\frac{24}{-1} = 24 \]
Therefore, the price at which Regina should sell her book to maximize her revenue is $24.