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As both the angles are complimentary 
so, let angle of depression is α
the angle of elevation = 90°- α
x= distance between man and pillar
tanα = 6/x
and also tanα = x/18
comparing both,
6/x= x/18
⇒x²= 18×6
x = 6√3 ft 
see the figure attached 
2 5 2