We are given an heptagon. NOte that we can divide the heptagon in triangles as follows
(The picture is an approximation).
This triangles are all the same , so to calculate the area of heptagon, we simply calculate the area of one of the triangles and then multiply it by 7.
We have the following triangle
REcall that the area of a triangle of base b and height h is given by the formula

By taking b=20.8 and h = 21.6, we get

So, the whole area would be

which rounded to the nearest square unit is 1572