Chapter 6 - Trigonometric Functions - 6.2 Trigonometric Functions: Unit Circle Approach - 6.2 Assess Your Understanding - Page 383: 146

$81\pi\ ft^2$

Step 1. The radius of the circular garden is $r=24/2=12\ ft$ Step 2. With the sidewalk, the larger circle will have a radius of $R=12+3=15\ ft$ Step 3. The area of the sidewalk is the difference between those of the two circles, we have $A=\pi (R^2-r^2)=\pi (15^2-12^2)=81\pi\ ft^2$