Remember that the answer is not only the yellow rectangle, but the combination of the two rectangles.  Have a look at this video:

Now what do you think?  Does it make sense that the two sub-rectangles are what they are?  

☐ No, it still doesn't make sense.

☐ Yes, because  21 + 6 = 15  

☐ I'd like to see the next video.

☐ I'd like to review an earlier video.