Solve the question..

Let the radius of the inner circle = r
Given :
Area enclosed between two concentric circles , A = 770 cm²
Radius of the outer circle, R = 21cm

solution :
Area enclosed between two concentric circles 'A' = Area of the Outer circle – Area of the inner circle
770 = πR² – πr²
770 = π(R² - r²)
770 = π(21² – r²)
770 = π(441 - r²)
770 = 22/7 (441 - r²)
770 × 7 = 22(441 - r²)
(441 - r²) = 770 × 7 / 22
(441 - r²) = (70 × 7)/2
(441 - r²) = 35 × 7
245 = 441 – r²
r² = 441 – 245
r² = 196
r = √196
r = 14cm  
Hence, the radius of the inner circle is 14cm.

hope this helps.