similar triangles means that if the angles are the same in both triangles, then the lengths of the corresponding sides relate to each other via the same multiplication factor.
in our example here, both triangles are similar, because they have the same angles.
the side of 6m length corresponds to the side of 72m length. that is a multiplication factor of 72/6 = 12.
now, also the other 2 sides would use the exactly same factor.
therefore, the height of the building is 2×12 = 24m.
