Answer:
11
Explanation:
In order to create a triangle, the two smallest sides must equal the largest side
Here we have 4 and 15. We could say 4+15=19, making 19 the third side.
But what if we made 15 the longest side
That would force us to make the 3rd side 11 instead because 4+11=15. And 11 is smaller than 19