How do historians use turning points to better orginize information about the past

They may base events from history on periods of time using a certain theme, so to say. This way, it allows them to organize information in a chronological order and base events on how severe they were and what led up to the most significant event in that time period, because a turning point is basically the most important part in a time period which initiated change. This also emphasizes on the importance of transitions.

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