Here are some things you can do to help guide you:
Calculate your elements of unsaturation, this should give you the number of double bonds or rings that are present. You know that there is a benzene, which is 4.
Permanganate breaks up double and triple bonds, or hydrates those bonds. The fact that you get 2 structures should indicate something. How many structures would you get if you used cyclohexene? If you take the two structures you get from the oxidation, and try and look at a way to combine them, there won't be many options available.
Two isomeric forms, what type of compound, related to the above information displays two different isomeric forms?
The information above is enough to generate the possible structure, the information about the dehydration is just fodder. After the structure, I can sit here and theorize why one isomer undergoes dehydation and the other doesn't, but it didn't really provide me any information leading to the solution (but it did help confirm I suppose).