y = -sin x - cos x

It says to use the reduction identity to graph each equation.

I have absolutely no clue how to do this. I recall my teacher saying he doesn't like to use the reduction identity given in the book so he changes the form to m*sin(x + y) and then use angle addition formula to get msin(x)cos(y) + mcos(y)sin(x). Then he states mcosy = -1 and msiny = -1. How did he get this? Because I have no clue how.