The transformation is a reflection on the y-axis.
If we take the y-axis as the mirror line as shown in the diagram below, we can see that the distance of vertices A, B, and C to the mirror line are equals to the distance of vertices A', B' and C' to the mirror line.
If we take the y-axis as the mirror line as shown in the diagram below, we can see that the distance of vertices A, B, and C to the mirror line are equals to the distance of vertices A', B' and C' to the mirror line.