Triangle abc has coordinates a(1,-1), b(0,2) and c(2,1) and it is reflecting over the line y = -x to form triangle a’b’c’ that are the coordinates of triangle a’b’c’

Reflecting over the line y = -x is equivalent to the transformation
  (x, y) ⇒ (-y, -x)

The coordinates of the image points are
  a'(1, -1) . . . . . . this point is on the line of reflection, so is invariant
  b'(-2, 0)
  c'(-1, -2)