Answer:
You negate the y-coordinate.
Step-by-step explanation:
A reflection about the x-axis will not change the x-coordinate of any point. This is because it goes straight across the x-axis; this does not change how much left or right of the origin the point is.
However, since each point is reflected across the x-axis, the y-coordinate will change. If it was above the x-axis, it will be below; if it was below the x-axis, it will be above. It will still be the same number of units away from the x-axis, however. This has the effect of negating the y-coordinate.
