Angle of sight from point A to tip of Mesa= 29 degrees
Angle of sight at point B to tip of Mesa = 25 degrees
Distance between A and B = 100 m
Let's height of Mesa from sight point be h and height of Stephen is 1.8 m.
With the point A traingle, h is derived as h = b x tan 29, b being the base.
With the point B traingle, h is derived as h = (b +100) x tan 25, b being the base
So b x tan 29 = (b +100) x tan 25 => (b x tan 29) - (b x tan 25) = 100tan 25
=> b = 100 (tan 25 / tan29 - tan25) => b = 100 (0.466 / 0.554 - 0.466) = 527.6
m
Height h = 527.69 x tan 29 = 292.5 m
Tallness of Mesa = h + 1.8 = 294.3 m