Here's what I've done:
Represent the set of 5 items by {a,b,c,d,e}. Assume that these items have already been arranged in ascending order.
The middle item, c, has the value 12.
Since "mode" means "most frequently appearing item," let's assume that d and e are both 15.
If the range is 9, we could then write 15-9 = 6, meaning that a = 6.
Then we have {6,b,12,15,15}
To find b, sum up {6,b,12,15,15}, equate the result to 11, and solve for b.