Question 1048398
(x + 21)^2 = x^2 + 1869 

X^2 + 441 = X^2 + 1869 ******* (x+21)^2 = x^2 + 42x + 441

x^2 + 441 - 441 = X^2 + 1869 - 441 

x^2 = x^2 + 1428