Question 515861
For any triangle on the plane, the length of any one side must be strictly less than the sum of the lengths of the other two sides.  This is called the Triangle Inequality.  It makes sense: the shortest path between two points should be a straight line, so it shouldn't be a shorter path to walk along the other two sides of the triangle.

In this case, we are given three side lengths: 9, 5, and 15.  Notice, however, that 15 is not strictly less than 9 + 5 = 14.  So this side is too long to make a triangle.  Thus a triangle with sides of length 9, 5, and 15 cannot be drawn.