Matt's Semicircles

Matt Enlow posed this puzzle - which I can't figure out. So I went to GeoGebra to try to build it.[br]https://twitter.com/CmonMattTHINK/status/1505595612967972873[br][br]Set the rectangle by moving B or setting the height and width. Adjust G until the semicircles' diameters align.

Information: Matt's Semicircles