The factor-of-ten relationship between the different units means we can combine the numbers in decimal fashion. If 1 unit is 1 zhang, then 1 chi is 0.1 zhang and 1 cun is 0.01 zhang. Hence 6 chi 8 cun is 0.68 zhang.
Let x and y represent the width and height, respectively. In terms of zhang, we have ...
y - x = 0.68
x^2 +y^2 = 1^2
Substituting y = 0.68 +x into the second equation gives ...