To reflect a point across the y-axis, you change the sign of the x-coordinate while keeping the y-coordinate the same.
For the point (3, 7):
- The x-coordinate is 3. When reflected across the y-axis, it becomes -3.
- The y-coordinate remains 7.
Therefore, the coordinates of the reflected point are (-3, 7).
So, the answer is: (-3, 7)