Height of the Lighthouse

Suppose a lighthouse is built on the top of a rock; the distance between a place of observation and that part of the rock level with the eye is 620 yds; the distance from the top of the rock to the place of observation is 846 yds; and from the top of the lighthouse, 900 yds.  Find the height of the lighthouse.

The Columbian Calculator, Almon Ticknor, 1846

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