Hello!
There are three possible lengths.
We need to keep in mind that the total area cant exceed [tex]60 ft^{2}[/tex], so, knowing that, we just need to find numbers that can be multiplied giving as result 60.
First combination:
[tex]4ft(height)X15ft(width)=60 ft^{2}[/tex]
Second combination:
[tex]5ft(height)X12ft(width)=60 ft^{2}[/tex]
Third combination:
[tex]6ft(height)X10ft(width)=60 ft^{2}[/tex]
Have a nice day!