∆abc is reflected about the line y = -x to give ∆a'b'c' with vertices a'(-1, 1), b'(-2, -1), c(-1, 0). what are the vertices of ∆abc?

Since A‘(-1,1) is on the line y=-x, reflecting it about the line doesn't change its coordinate, so A's coordinate is (-1,1). Reflecting B'(-2,1) about y=-x switches both the sign and the number of x and y, so B=(-1,2), and C'(-1,0) becomes C(0,1). Drawing out the graph with the points and the line helps.

What do you mean i don’t understand