The graph for 'y = -x' (same as 'x = -y') is a streight line.

To draw a streight line you need to points from the line.

You can read the points from the graph or you calcuate:

For x = 0 

y = - 0

y = 0

One point is (0, 0)

For x = 1

y = - 1

Another point is (1, -1)