Answer:
Any real number strictly larger than 9 and strictly less than 21.
Step-by-step explanation:
By triangle inequality, the sum of the lengths of any two sides are greater than the length of the third side.
We therefore could set up three inequalities.
x+6>15, x+15>6, 15+6>x
Solving these, we get x>9, x>-9, x<21 i.e. 9 < x < 21.
