The diagonal of a rectangle is 20 cm and its perimeter is 56 cm. The length of the rectangle is:

With length $l$ and breadth $b,l+b=28$ and $l^2+b^2=400$. Then $2\text{lb}=(l+b{)}^2-(l^2+b^2)=784-400=384$, so lb = 192. Hence $l$ and $b$ are the roots of $t^2-28t+192=0$, giving 16 and 12. The length is 16 cm (and $16^2+12^2=400$).