The area of the sidewalk is 131.88 sq. m.
First find the area of the bed with the sidewalk. Since the sidewalk is all around the circle, this will increase the diameter by 4 m, making d=23. The area of the circle is given by
A=πr²
The radius would be 23/2 = 11.5:
A=3.14(11.5)² = 415.265
Now find the area of the flower bed without the sidewalk. Since d=19, r=19/2=9.5:
A=3.14(9.5²) = 283.385
Subtract the two:
415.265-283.385 = 131.88