How and why was the Great Wall built?

Builders of the wall always tried to use local resources, so the walls that crossed mountains were made from stone, and the walls that crossed the plains were made from rammed earth. The much later Ming Dynasty built a stronger wall by using more bricks and stone instead of rammed earth like some of the first. Great Wall has been built and rebuilt for three main purposes: as kingdom border defenses, to defend China's northern border, and for tourism.

Explanation:

the perimeter of a triangle is 69 cm. side a is 5 cm shorter than side b. side c is twice as long as side a. find the length of each side. i need to know how to do it in 2 equations.  

equations:

a = b-5

c = 2a

a + b + c = 69

whats the video? can u send a link?