Answer: -3x+3y = 9
This line passes through points A and C.
=============================================================
Explanation:
We can rule out choice A because this horizontal line does not pass through the center of the square. This line only passes through points B and C. There's no way it can be a reflection to have ABCD map onto itself.
Through trial and error, you should find that the answer is D.
Note that the line through points A and C passes through the center of the square located at (-2,1)
The equation of line AC is y = x+3 which rearranges to -x+y = 3. If we tripled both sides, then we end up with -3x+3y = 9. It's not clear why your teacher decided to triple both sides like this.
If we reflected the square over the line mentioned, then points B and D will swap places. Points A and C will stay where they are. Overall, square ABCD will map onto itself. This is another way of saying no change has occurred assuming we erase the point labels and just focus on the square itself.
