"Similar" means the sides of one figure have the same ratio as the corresponding sides of the other figure. We see that AB = 2×CD. JK corresponds to AB, and LM corresponds to CD.
__
We could bother to calculate that sides of JKLM are 1.2 times as long as the corresponding sides of ABCD, but that is not necessary. We only need to find a convenient ratio between the side we want and some other known side. I like a ratio of 2, because it is easy to double a number. We find AB=2CD, so JK=2LM=2×2.4 = 4.8.
