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

Let S, M, L represent the shortest, mid-length, and longest sides, respectively. You are given three bits of information.
.. S +M +L = 14.5 cm
.. L = 2S
.. L = 6.2 cm
You can use an equation something like this to find the middle-length side.
.. (6.2 cm)/2 +M +6.2 cm = 14.5 cm
.. M +9.3 cm = 14.5 cm . . . . . . . . . . . . . constant terms collected
.. M = 5.2 cm . . . . . . . . . . . . . . . . . . . . . subtract 9.3 cm

Now, you know the side lengths are 3.1 cm, 5.2 cm, 6.2 cm.

You have not offered any choices for "which equation," so take your pick of those used here.