To find the coordinates of point \( D' \) after reflecting point \( D(5, 6) \) over the \( y \)-axis, you need to change the sign of the x-coordinate while keeping the y-coordinate the same.
The coordinates of point \( D \) are \( (5, 6) \). After reflection over the \( y \)-axis, the new coordinates \( D' \) will be:
\[ D' = (-5, 6) \]
So, the coordinates of point \( D' \) are \( (-5, 6) \).