The perimeter of a scalene triangle is 14.5 cm. the longest side is twice that of the shortest side. which equation can be used to find the side lengths if the longest side measures 6.2 cm?

This question has already been solved by another person. I'll list their answer here. 

Perimeter is the sum of all sides of a polygon. For this case, the perimeter is expressed as: P=a+b+c. It has been said that the longest side is twice that of the shortest side. Let's assume that a is the longest side and b is the shortest side. So, the equation for the shortest side is, 

6.2 = 2c
c = 3.1 

14.5 = 6.2 + 3.1 + b
14.5 = 9.3 + b

I take no credit for this answer. It was solved by Taskmasters. 

We need more info then that