Question

What is the area of 11 m 21 m 8 m 5 m

Answer 1

Here's how to find the area of an irregular polygon.

http://www.mathopenref.com/polygonirregulararea.html

Answer 2

And, to make things worse, just having the side length does not determine the area. Take a square. It can be squashed flat, giving area from s^2 down to 0.