Let's go:
There will only be a triangle if only its sides obeyed the following rule: one of its sides must be greater than the absolute value (modulus) of the difference of the other two sides and less than the sum of the other two sides:
|a - b| < c < a + b
|a - c| < b < a + c
|b - c| < a < b + c
|16 - 5| < x < 16 + 5
11 < x < 21
I hope I helped you.