That looks like "half" of the proof using a square:

https://www.onlinemathlearning.com/image-files/xpythagorean-...

where you draw three extra triangles, not just one, and they surround a square of c x c. Think about it as making two copies of the trapezoid, one rotated on top of the other.

I shared this one with my son, the step where the 2ab expressions cancel out gave him a little aha moment.