what is the height of a triangle whose sides are 13 , 20 and 21 cm

Answer 1

Use this formula:
height=2*K/base

Answer 2

The given sides look suspiciously like the sides of a Heronian triangle, one that is comprised of two Pyhagorean triangles with a common altitude. In reality, the triangle is the 5-12-13 triangle joined to the 12-16-20 triangle having the common altitude of 12.