We can use pythagorean theorem to solve this problem; pythagorean theorem is denoted by the formula a^2 + b^2 = c^2. We already have b, as 21 yd and c, as the hypotenuse is 35 yd. We can plug these values into the formula:
C is the longest side which is 35 ye and b is 21 yd. Then you would subtract 21^2 from 35^2 and you would get 784. Then find the square root of 784 and you would get 28 as the answer! Good luck and have a great day or night!