Answer:
x = -2
Explanation:
The line of reflection is a line about which you can rotate of reflect to get a mirror image.
In our case, looking at the triangles ABC and A'B'C' we have to ask ourselves "about which line should I rotate or reflect ABC so that I get A'B'C'? "
The answer is if you flip ABC about the line x = -2, you will get A'B'C'.