Triangle is isosceles , it means two sides of the triangle are equal
and perimeter = 14cm
Let the two equal sides = x cm each
One of its sides is twice longer than another of its sides.
It means Third side = 2x
Now perimeter - 14 cm
It means sum of all sides = 14 cm
So
x+x+2x = 14
4x=14
Divide both sides by 4
x= 3.5 cm
It means two equal sides are 3.5 cm each
and third side = 2(3.5) = 7 cm
So three sides of triangle are 3.5 cm , 3.5 cm , 7 cm