I think you meant 125 dm³ since it's volume so i presume that's just a clerical error
Remember that the volume of a cube is just the cube of the side or
V = s³
where V is the volume and s is the length of one side..
substituting to get s, you'll have
125 = s³
s = ∛125
s = 5
since you are asked the length of the frame, try to examine a cube. notice that there are 12 edges. so when framing it you'll need 12 of s.
so you'll have the total length as:
Total length = 12s
                  = 12(5)
                  = 60 dm