What change did the mongols bring to china

The Mongol Empire under Genghis Khan started the conquest with small-scale raids into Western Xia in 1205 and 1207. By 1279, the Mongol leader Kublai Khan had established the Yuan dynasty in China and crushed the last Song resistance, which marked the onset of all of China under the Mongol Yuan rule.

Explanation:

In the following decades, the Mongols regularly found ways around or through the Wall. In 1279, the Mongols—under the leadership of Kublai Khan, Genghis' grandson—defeated the Song and ruled all of China for the next 100 years (the Yuan Dynasty).

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

rescuing someone who is in danger. hope this : )