2(ha² + hb² + hc²) = a² + b² + c²
The a, b, c attached to the h’s should be subscripts. In the regular heptagon and the heptagonal triangle, a = side length, b = length of short diagonal, c = length of long diagonal, R = length of circumradius, ha = altitude off side a, hb = altitude off side b, & hc = altitude off side c.