If d is the midpoint of the segment ac and c is the midpoint of segment db , what is the length of the segment ab , if ac = 3 cm.

Comment
The only way I can envision this is put all 4 points on 1 line segment. They go in the order of ADCB. I can only draw it this one way.

Given
D is the midpoint of AC
C is the midpoint of DB
AC = 3 cm

Argument
AC = 3 cm           Given
AD = DC                       D is the midpoint of AC
AD = 3/2                       Definition of a midpoint.
DC = 3/2                       Definition of a midpoint

DC = 3/2                       Last statement
DC + CB = 1.5 + 1.5.   C is the midpoint of BD
DB = 3                          Consolidation of equals
AD + DB = AB               
1.5 + 3 = 4.5                 Substitution.