Since the triangles are similar the ratios between corresponding sides are the same.
For example, the factor that gets you from GH to DF will be the same factor that get you from HE to FE and GE to DE.
This factor is DF/GH=12/8=1.5.
Then, FE = HE * 1.5 = 22.5 and DE = GE * 1.5 = 25.5.
