A= 28.
The formula for the Pythagorean theorem is A^2 + B^2 = C^2
So plug in what you know. A^2 + 21^2 = 35^2 you get A^2+ 441 = 1225. You then take away 441 from 1225 and you get A^2 = 784. You have to find the square root of 784 which is 28. So side A is 28.
