Let's construct a rectangle, given one side (b), as well as the difference between the diagonal and the other side $(d-a)$.
The only difference from the previous one is that now in the right-angled triangle $B C A_{1}$, $B A_{1}=d-a$. If we construct point $A$ in the same way, then

$$
C A - A B = A A_{1} - A B = B A_{1} = d - a
$$

thus $A B$ is the side $a$ of the desired rectangle.

(Pálos Tibor, Budapest.)