When a point \((x, y)\) is reflected across the y-axis, the x-coordinate changes sign while the y-coordinate remains the same.
For the point K with coordinates \((6, 3)\):
- The x-coordinate \(6\) becomes \(-6\).
- The y-coordinate \(3\) remains \(3\).
Thus, the new coordinates after reflection across the y-axis will be \((-6, 3)\).
So the correct response is:
\((-6, 3)\)