A tetrahedron is a triangular pyramid. It has four triangular faces and four vertices.
In the question, we are told that the length of the edges are all equal = 20m and the length from one corner to the center of the base = 11.5m
We sketch a diagram of a tetrahedron with the given measurement as shown below
To find the height, we will use the Pythagoras theorem
20² = h² + 11.5²
h² = 20² - 11.5²
h² = 267.75
h = √267.75
h = 16.36 (rounded to two decimal places)