I suggest you cut out the figure on the right from a piece of paper and fold it to look like the figure on the left. This can give you the experience you need to do these in the future.
It is about pattern matching. The only right triangle in the figure on the left has a long side of 15, a short side of 8, and a hypotenuse of 17.
The right triangle in the figure on the right has a hypotenuse of D (so D=17), a short side of C (so C=8). If you fold the figure, you find that B is the long leg of the right triangle (B=15).
All of the rectangular shapes in the figure on the left have width 4. All of the rectangular shapes in the figure on the right have width A (A=4).
The mapping you desire is
.. A = 4
.. B = 15
.. C = 8
.. D = 17
