Mathematically, reflection across the x-axis means a point (x,y) undergoes a change to the y point, (x,-y).
Examples:
Point α is (4,6), find the relfection across the x-axis of Point α...
The reflection of Point α across the x-axis is (4,-6)
Point γ is (-420, -1337), find the reflection across the x-axis of Point γ...
The reflection of Point γ across the x-axis is (-420, 1337)