To find the new y-coordinate of point D after the translation described by the function \( (x, y) \rightarrow (x + 6, y - 4) \), we start with the original coordinates of point D, which are \( D(-2.5, 5) \).
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Translate the x-coordinate: \[ x' = -2.5 + 6 = 3.5 \]
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Translate the y-coordinate: \[ y' = 5 - 4 = 1 \]
After the translation, point D becomes \( D'(3.5, 1) \).
Thus, the y-coordinate of point D after the translation is \( 1 \).
So, you can fill in the blank:
D′(3.5, 1).