we know that
area of a circle=pi*r²
diameter=10 m
radius r=10/2-----> 5 m
area of a circle=pi*5²----> 78.5 m²-----> area of the circular garden
area of a square=b²
where b is the length side of the square
b=10 m
area of a square=10²-----> 100 m²------> area of the square shaped yard
so
the area of the yard that will not be covered with mulch=100-78.5
the area of the yard that will not be covered with mulch=21.5 m²
the answer is
21.5 m²