Graph with vertices r(6, 6), s(3, –6), and t(0, 3) and its image after a reflection over the y-axis.

If the reflection line is the Y axis, change (x, y) by (-x, y)
 We have then:
 R (6, 6) ---> R '(- 6, 6)
 S (3, -6) ---> S '(- 3, -6)
 T (0, 3) > T '(0, 3)
 
 The new vertices are
 R '(- 6, 6)
 S '(- 3, -6)
 T '(0, 3)
 Graph four (last from left to right) is the correct one.

Agc and bgd are vertical angles