The area of any square is

Where "s" is the length of the side of the square
a)
The given triangle has:
Hypotenuse is c
Two legs of the right angle are 21 cm and 28 cm
The area of the square of side length c is

Then we will use the Pythagorean relationship to find c^2

Then the area of the square of side c is 1225 square cm
b)
The given triangle has:
Hypotenuse of length 13 mm
Two legs of the right angle are 5 mm and c
The area of the square of the side length c is

We will use the Pythagorean relationship to find it

Subtract 25 from both sides

The area of the square of side length c is 144 square mm