Answer:
6 < c < 36
Step-by-step explanation:
We can call the 3rd length c, and the length of this side can not be smaller than 21 - 15 = 6 cm because then the two sides would not meet, and the same for 21 + 15 = 36, where c has to be smaller than 36, because then the other 2 sides would not meet.
