Given y=x if linear function is refelected over y-axis what is the new linear function?

Reflections across the y axis change the point  (a , b) on the original graph to the point (-a, b) on the reflected graph.

Thus, the equation of y = x becomes  y = -x 

it equals 2

If the line  y = x  is reflected through the y-axis it becomes  y = -x.