Notice that TV is in the original triangle and in the image. That means that we are looking for a transformation where this segment remains fixed (doesn’t move) which is the case when doing a reflection. A reflection over the line segment TV would leave that segment unchanged. Think of having the triangle iba page and picking p vertex S swinging it over TV and having it land on the other side. TV the line of reflection remains fixed while S moves to a new point here called U.
