Most likely, polygon ABCD has sides of known lengths.
It is also likely that one of the sides of polygon EFGH (not EH) is also known. For instance, its side EF.
If the above is true, we can find the scaling factor as a ratio between lengths of corresponding sides:
r=EFAB
Since this ratio is constant for any two corresponding lengths,
r=EHAD
From the last two equations we can derive:
EH=AD⋅EFAB
Hope That Helped : ) (Took a minute)