Explanation:
E = (-8, 2)
reflected across the y axis means that there is kind of a mirror along the y-axis.
and when you look into that mirror, you see point D as point E all the way on the other side of the mirror, at the same distance from the mirror as point D is. and point E has the same "height" (y) as point D.
since the other side of the mirror represents the negative numbers, the same distance to the mirror means the same amount of negative units away from the mirror for E, as we have positive units away from the mirror for D.