Reflection Over the Y-axis
When a function is reflected over the y-axis, its graph is mapped to the inverse x-axis as if it was a mirror.
Concretely, every point (x, y) of the original graph maps to (-x, y). The y-coordinates are preserved and the x-coordinates are inverted, thus the relationship between the two functions graphed in the image is:
f(x) = g(-x)
Or, equivalently:
g(x) = f(-x)