Doina Popovici's video explains how to calculate the area of a ring. She takes us really slow, down to all the steps of this process. It's very important that she explains every math formula and operation so you cannot get lost into this solving. The video uses nice pictures and colors in order to make learning easier and nicer for those who are interested. At the end of the video you'll surely be able to solve by yourself a problem like that and your appetite for geometry will grow significantly!