The tangent of an angle is the ratio of the opposite side to the adjacent side.
... tan(45°) = (height above eye level)/(20 m)
... height above eye level = (20 m)·tan(45°) = (20 m)·1 = 20 m
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Eye level is 1.7 m above ground, so the height of the flagpole above ground is
... height of pole = height above eye level + height of eye level
... height of pole = 20 m + 1.7 m = 21.7 m
