What are the important cities of the byzantine empire?

Alexandria, Antioch and Jerusalem, which were lost to the Arabs. And Thessaloniki which was the largest Greek city after Constantinople, until Paris grew larger as a Western university city in high medieval times. By then East Rome had lost its glory, and was surpassed by Venice, Genoa, Milan and Florence.

the social contract theory

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