If ive got y = f(x), is the transformation to y = f(-x) a reflection in the x-axis or the y-axis?

Answer 1

It's y-axis reflection, as if the vertical y-axis were a mirror.

x-axis reflection would be y=f(x) and y=-f(x)

Answer 2

Y axis
Think about it this way. The Positive X is on the right side of the Y axis, the negative X is on the left. To change the positive to a negative you'd have to go over the Y axis

Answer 3

The y-axis. The images of y = f(-x) and y = f(x) symmetric with respect to the y axis. The reflection around the x -axis gives the function y = - f(x)