We have that the length of 1 leg is twice that of the base, and hence, the length of the other leg is also twice the length of the base.
Therefore by setting the length of the base as x we have that both legs have length 2x.
Therefore as the perimeter is the length of all the sides we have that 20cm = x + 2x + 2x = 5x
5x = 20
x = 4
The length of the base is 4cm