Here's how to find the area of an irregular polygon.
http://www.mathopenref.com/polygonirregulararea.html
What is the area of 11 m 21 m 8 m 5 m
2 answers
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.