When reflecting across the Y axis, the Y values remain the same.
Now if you were reflecting across Y = 0, the x values would just be inverse ( opposite signs).
So this triangle if reflected across Y = 0 the new vertices would be (4,4) (2,3) and (5,2)
Now since the reflection line is y = -1, which is a one unit shift to the left of y = 0, subtract 1 unit from each X value.
The locations are now: A'(3,4), B'(1,3) and C'(4,2)