When two shapes are mathematically similar, that means that all their sides are in a proportion.
We know the sieds of EH and AD, and so we can find the proportion of the shapes using these two sides.
EH/AD = 6/9 = 2/3
The ratio of EFGH to ABCD is 2:3
Now we have to find BC.
Becuase they're in proportion, we can form this equation:
=
Cross multiply:
24 = 2*BC
Divide both sides by 2:
BC = 12 cm
Good luck!