Question 1064972
<pre><b>
The green line is the line y = -x.
We are to reflect the triangle and its three vertices
about the green line.   

{{{drawing(400,400,-3,3,-3,3, grid(1),

green(line(-10,10,10,-10)), circle(-1,1,.05),circle(-2,-1,.05),circle(-1,0,.05),circle(-1,1,.1),circle(-2,-1,.1),circle(-1,0,.1),
circle(-1,1,.025),circle(-2,-1,.025),circle(-1,0,.025),
circle(-1,1,.075),circle(-2,-1,.075),circle(-1,0,.075),
circle(-1,1,.065),circle(-2,-1,.065),circle(-1,0,.065),


red(line(-1,1,-1,0),line(-1,1,-2,-1),line(-2,-1,-1,0)),
locate(-1+.1,1+.28,"A'"),locate(-1+.1,0+.32,"C"),
locate(-2+.07,-1-.07,"B'")

)}}} 

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:

{{{drawing(400,400,-3,3,-3,3, grid(1),

green(line(-10,10,10,-10)), circle(-1,1,.05),circle(-2,-1,.05),circle(-1,0,.05),circle(-1,1,.1),circle(-2,-1,.1),circle(-1,0,.1),
circle(-1,1,.025),circle(-2,-1,.025),circle(-1,0,.025),
circle(-1,1,.075),circle(-2,-1,.075),circle(-1,0,.075),
circle(-1,1,.065),circle(-2,-1,.065),circle(-1,0,.065),

circle(-1,1,.05),circle(1,2,.05),circle(0,1,.05),circle(-1,1,.1),circle(1,2,.1),circle(0,1,.1),
circle(-1,1,.025),circle(1,2,.025),circle(0,1,.025),
circle(-1,1,.075),circle(1,2,.075),circle(0,1,.075),
circle(-1,1,.065),circle(1,2,.065),circle(0,1,.065),


red(line(-1,1,0,1),line(-1,1,1,2),line(1,2,0,1)),

red(line(-1,1,-1,0),line(-1,1,-2,-1),line(-2,-1,-1,0)),
locate(-1+.1,1+.28,"A'"),locate(-1+.1,0+.32,"C"),
locate(-2+.07,-1-.07,"B'")

)}}} 

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</pre><b>