Respuesta :

If the third side (X) is the hypotenuse then length of X is sqrt(20^2+21^2)
If the hypotenuse is the side 21 cm long then length of the third side = sqrt(21^2-20^2) . I hopr this helped!!
leeandrews2
A^2 + B^2 = C^2

21^2 + 20^2 =c^2

441 + 400 = c^2

841 = C^2  take the square root to get your answer.

c = 29