Well... Basically, you should prove this by SSS property(side-side-side). It's fair to say that the length of a side is equal to itself so The line that cuts through the rectangle is a side for both rectangles. Thus because of the given, all the sides of the triangles are equal to one another. This is a very important trick for geometry(I remember using it a lot). Hope this helps!
Inscribed is a polygon inside a circle with all points on a given point in the circle. Circumscribed is a circle inside a polygon with any given point touching just one point on the polygon. Hope this helped.
