What are mapmakers referring to when they talk about balance?

When mapmakers talk about balance it is about element placing. They need to make a map that is not crowded but also not full of blank space or any blank space at all. This leads to them excluding many elements of a given region just because there is not enough space on the paper to put it on. In order to put more elements on a map, map creators create maps of different scales and regions so that more details could be incorporated into a map.

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