The triangle inequality tells us that the sum of the two shorter sides of a triangle must be greater than the third side.
Here, 21 cm is one of the shorter sides, and x cm must be the length of the other shorter side.
So using the triangle inequality, 21+x > 2x or x<21 cm To the nearest 0.1 cm, the greatest possible length of the longest side (2x) is 2*21-0.1 cm = 41.9 cm < 2x=42 cm.
