Squares / rectangles are simple, just multiply one side by the other. Answer will always be written like 8cm2
Triangles are pretty easy to. You do exactly the same thing as a square by you half the answer. The reason is a triangle will ways fit into a rectangle and take up exactly half of the room.
With regard to the pentagon, it depends what information you are given about it.
Also, if you're going to be asking more questions in here, don't keep editing the first post, just ask the question in a new reply, it makes it much easier for you and the people replying.