Question 407613
Let x = The Length of the Rectangle. Given the Width then
the equation relating Length, Width and Diagonal is 
       Width^2 + Length^2 = Diagonal^2
       21^2 + x^2 = 34^2
       x^2 = 34^2 - 21^2
       x^2 = 1156 - 441
       x^2 = 715
       x = 715^(1/2)
       x = 26.7394
       The Length of the Rectangle = 26.7394