You can put this solution on YOUR website! It wasn't invented. It was more like discovered because topics in math and science aren't something you invent. You discover them.
As for who did this, it's very hard to say because area dates as far back to ancient Egypt and possibly even further back in time. So whoever discovered the idea that area of a rectangle is base times height was some ancient thinker who didn't put down his/her name or their original work is lost.