Explanation:
as the height in such a triangle (all 3 sides are equally long, and therefore also all 3 angles are equal 60°) is creating a right angle (90°) with the opposing side (and "splitting" it in half at the crossing point), we can use Pythagoras to calculate the length of the height :
side is the baseline, the height is one leg, and half of the other side is the second leg.
18² = height² + (18/2)² = height² + 9²
324 = height² + 81
height² = 243
height = 15.58845727... cm ≈ 15.6 cm