21, 28, and 36 units is a triangle

Yes, it could be a triangle.

Step-by-step explanation:

If the Pythagorean Theorem is satisfied, then these three side lengths represent a right triangle:

21^2 + 28^2 = 36^2 = > 1225 = 1296 FALSE; we don't have a right triangle here.

However, the sum 21 + 28 is greater than 36, so we could indeed construct a triangle from these 3 side lengths. It just wouldn't be a right triangle.