With the given numbers, you can already assume what the other two lengths are. The shorter length of 3m will typically be the length of the shorter side or the height while 9m will typically be the width.
With this information, you have all your sides now - both of the shorter sides are 3 and the longer sides are 9.
To find perimeter, you now add all of the sides together which results in 24m being you're perimeter.
Hope this helps!