The green line is the line y = -x.
We are to reflect the triangle and its three vertices
about the green line.   

 

One of those points is ON the green line, so it will 
reflect into ITSELF!  The other two will reflect on the
other side of the green line like this:

 

All that's left for you to do is to give the coordinates
of the reflected points, which I am sure you can do
without my help.

Edwin