Question 536494
This problem requires understanding right triangles.  Take the words out and picture a triangle in which the base = 200 ft (the distance the kite is behind the boat) and the hypotenuse is 250 ft (the length of the string holding the kite).  We are left to figure out the other side, which is the height.
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The basic formula linking the 3 sides of a right triangle is the pythagorean theorem: c^2 = a^2 + b^2, where, c is the hypotenuse, and a and b are the base and height of the triangle.  We know c and a, so we need find b, and to do that, we need to rewrite the pythagorean theorem in terms of b:
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c^2 = a^2 + b^2 is the same as:
c^2 - a^2 = b^2 after subtracting a^2 from both sides
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flip it around: 
b^2 = c^2 - a^2
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take the Sqrt of both sides:
b = Sqrt(c^2 - a^2)
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b = Sqrt(250^2 - 200^2)
b = 150
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The kite is 150 ft above the water
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Cheers,
Lee