Chapter 5 - Trigonometric Functions - Section 5.2 Trigonometric Functions: Unit Circle Approach - 5.2 Assess Your Understanding - Page 406: 146

$81\pi\ ft^2$

1. With diameter $d=24\ ft$, we have its radius $r=12\ ft$ for the circle of the garden. 2. Adding the sidewalk will form a new circle with radius $R=r+3=15\ ft$. 3. The area of the sidewalk is the difference between the two circles, we have $A=\pi R^2-\pi r^2=\pi (15^2-12^2)=81\pi\ ft^2$