X+1<

21

_

4 or the fractions 21 4

Step-by-step explanation:

Triangles have one useful property that lets us identify them from a set of three side lengths: any pair, when added up, will equal a larger number than the third.  So:

7+9=16 (which is larger than the other number, 12) 
7+12=19 (larger than 9)
9+12=21 (larger than 7)

So the answer is True.