Question

find the image of S(-1,-3) after a reflection scross the line y=-2

Answer 1

The best way to do this is to actually sketch it.
I am sure it will make perfect sense to you after you draw it.
Remember that y =-2 is a horizontal line 2 units below the x-axis

Answer 2

the image is the same distance above the line y = -2 as it was below it to start. So,

y -> -2+(-2-y) = -2+(y+4) = y+2
(-1,-3) -> (-1,-1)