@angelicapad All sides are equal for an equilateral triangle, altitude for an equilateral triangle is also the median. So altitude will the half the side. See the diagram|dw:1352702635737:dw| all sides are equal let, them be a and we have the altitude as $$7\sqrt 3$$ In triangle ABD AB=a AD=$$7\sqrt 3$$ BD=a/2 ABD is a right triangle, can you find a now?