Consider the triangles formed by the lines of sight in parts a and b. In part a, the distance from the horizontal to the base of the antenna is 82.0tan31.0º; in part b, the distance from the horizontal to the top is 82.0tan34.2º. The antenna height is the difference between these
h = 82.0tan34.2º - 82.0tan31.0º = 82.0*(tan34.2º - tan31.0º) = 6.46 m