Question

Is it possible to construct a triangle with side lengths of 11 cm, 15 cm, and 21 cm? Why or why not?

Answer 1

There is only one requirement for three sides in order to be considered possible in constructing a triangle. That requirement is
The sum of any of the two sides must be greater than the third side

11 + 15 = 26      >      21
11+ 21 = 33       >      15
15 + 21 = 16      >      11

Since all of the combination fulfill the requirement, yes we could create the triangle with those sides.

Answer 2

Since all of the sums are greater than the third side, it is possible to make a triangle with these three sides.