(Optimization) Whats making this rectangle have an area of (2)xy??

https://preview.redd.it/9ddcviiskhng1.png?width=779&format=png&auto=webp&s=05b3d8a0ef8f28fa434d480c63f091bb72d9f5e1

From my understanding its because the rectangle is on the negative side and positive so its something like x--x= 2x, i dont get why or how we do that?

Whats the difference between this rectangle and a normal one where we just do A= bh, whats the overall reason the rectangle is getting split?