Answer: 13 cm
The triangle ABC is isosceles, so it has two equal sides. ( line BC = line AC )
Line AC = 3.5cm
Line JL of triangle JKL is twice as big as its corresponding part in triangle ABC, so we can say that the triangles have a ratio of 1:2
Line BA = 6.5cm
so
Line KJ = 13cm
Have a good day ^^