The figures are similar, so the corresponding sides form a ratio of 3/2
AB corresponds to EF (AB is the first two letters of ABCD; EF is the first two letters of EFGH)
Set up the following equation and isolate EF through cross multiplying
AB/EF = 3/2
18/EF = 3/2
18*2 = EF*3
36 = 3*EF
3*EF = 36
3*EF/3 = 36/3
EF = 12
Answer: Segment EF is 12 units long