Can someone help with this math's question?

My aim is to find the area of a polygon. I have a regular octagon that has a side length of 2n. Perimeter = 2nx8 - (8 sides). The octagon has been split into equilateral triangles (I would suggest drawing this for yourself). Inside one of the triangles in the bottom left hand corner it has the angle 67.5 degrees. What formula would I use to figure out the area? Thanks alot.