Try this: Combine 6 and 9, obtaining 15.
Compare this sum, 15, to the length of the 3rd side: 15.
If placed end to end, the 2 sides with lengths 6 and 9 form a new line of length 15. We'd end up with this new line superimposed on the 3rd side (with length 15).
Think: Do 2 straight lines of length 15, superimposed on one another, form a triangle?
