In figure A:
21 ^2 + 28^2 = C ^2
928 = c^2
therefore side c = 30.36cm
since we need to find the area of a square inscribed in a triangle , we have a right triangle with sides 21cm, 28cm and 30.46cm :
3/4 c + c +4/3c = 30.5
37/12 c = 30.5
c = 30.5 *12 /37
c = 377/37 =9.89
Therefore :
Area of a square = 9.89^2= 97.8 cm^2