If you want to reflect over the y axis then your answer would want to be going either from the left side to the right side, or right side over to the left side. If the figure was on the top or bottom you would go over the x axis.
So to reflect over the y axis you must find the points that each angle or corner of the figure your reflecting. You'll want to remember those locations of the points, then you'll transfer them over to the other side on the same points but on the other side. I'll make an example to make it more clear
For the first number, before the coma, if the number is negative you'll go left, if the number is positive you go right
For the second number, after the coma, you go up if it's positive or down if its negative.
If A is located at (-3, +2)
Angle B at (-1, +2)
Angle C at (-2, +0)
To transfer them over the y axis the numbers being positive or negative will change
For example
Angle A will become (+3, +2)
Angle B will become (+1, +2)
And angle C will become (+2, 0)
If the number is zero then it stays on the default line.
Hope this helped,reply if you need more help!!