Question 940670
Let x be the third side. Then


x+7 > 14
7+14 > x
14+x > 7 (this is obviously true so ignore it)


The first two inequalities are equivalent to x > 7 and x < 21.


In general, if a and b are two sides of a triangle, with a <= b, then the third side c is in the range (b-a, b+a).