Since this makes use of the Pythagorean theorem, you've probably run across the 3-4-5 triangle. You often find scaled versions of it in algebra and geometry problems. Here, it is scaled by a factor of 4 to give a 12-16-20 triangle as half of the display screen.
The 3-4-5 triple is the only Pythagorean triple that is an arithmetic sequence. So, if the difference in side lengths is 4 and the diagonal is 5×4, you can be pretty certain that x = 4×4 and x-4 = 4×3.