The line of reflection is what the graph flips over. You can find the line with two points, and a point on the reflection line is the midpoint of a point and the corresponding point in the after-image.
The first one reflects over the y-axis, or x=0. One point is (-2, 1) and its corresponding point is (2, -1). The midpoint is found by the average of the two coordinates, which is (0,0). Pick another pair of points and find the midpoint which you should get (x,0).
You have two points (0,0) and (x,0) and they form a line, which is the y-axis, or x=0.
The line of reflection for the 1st one is x=0 (y-axis).