This seems to be a reflection over the 'y' axis. Notice that the 'x' coordinates of points U and V are x =2 in the original figure, and then, 'x' coordinates of U' and V' are x = -2
All 'y' coordinates for the transformated figure (S'T'U'V'W') are the same that the original figure (STUVW), but the 'x' coordinates are exactly the opposite
This transformation is :