To rotate the line segment with endpoints \( (0, -3) \) and \( (0, -7) \) 90° clockwise, we will first analyze the endpoint that we will rotate, \( (0, -3) \).
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The general rule for rotating a point \( (x, y) \) 90° clockwise around the origin is given by the transformation: \[ (x, y) \rightarrow (y, -x) \]
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Apply this transformation to the point \( (0, -3) \): \[ (0, -3) \rightarrow (-3, 0) \]
Thus, the endpoint \( (0, -3) \) when rotated 90° clockwise becomes \( (-3, 0) \).
We can also apply the same transformation to the other endpoint \( (0, -7) \): \[ (0, -7) \rightarrow (-7, 0) \]
However, the question specifically asks for an endpoint of the rotated segment (which includes the transformed endpoint of \( (0, -3) \)), therefore the answer is:
\[ \boxed{(-3, 0)} \]