16. Ivan Desaacs is 6 feet tall and walks towards a tree that is 21 feet tall as shown in the figure. Let x represent the length of Ivan's shadow and let y represent the distance Ivan is from the base of the tree at time t. The distance between Ivan and the tree is changing at a rate of 3 feet per second.
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a) At what rate is the length of Ivan's shadow changing when y = 25 feet?