Which condition made the western roman empire vulnerable to attack?

I believe the answer is: Splitting of empire; Weak borders

The splitting empire happened because of the power struggle among the kings' Sons. The split resulted in the reduction of Roman military power which made them become weaker compared to other empires. Weak border provide opportunities for Barbarian tribes to come in an raid Roman villages.

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