Since the video you reference deals with regular polygons I will do this problem as a regular heptagon.
The formula for area of a regular polygon is (1/2)ap where a stands for the apothem (the perpendicular distance from the center of the polygon to one of its sides) and p stands for the perimeter.
To find the apothem, you need to know how to find the tangent of an angle in a triangle.
First, divide the heptagon into 7 triangles by drawing a line from the center to each of the vertices.
Draw an altitude for one of the triangles- this is your apothem.
You have created a right triangle. To find the angle whose vertex is at the center of the heptagon divide 360/14 to get 25 5/7.
The bottom of the right triangle is found by dividing the perimeter by 7 and then by 2 : 210/14 = 15
Solve tan(25 5/7) = 15/x
x = 22.8
Perimeter = 1/2 x 22.8 x 210 = 2395.88 (answer without rounding in between steps)