Part 1
Determine the area of the square
The area of the square is

where
b is the length side of the square
In this problem, the value of b is equal to the diameter of the circle
so
b=2(21)=42 cm

Part 2
Find out the area of the shaded region
The area of the shaded region is equal to the area of the square minus the area of the circle
so

we have
pi=22/7
r=21 cm
substitute

The answer is 378 cm2