I am asked to find the equation of a line with a slope of -1 that is tangent to the curve

I know I am doing something wrong here, but I just can't figure out what.

first, I have the equation for the slope, y=-1x+k

next, I set the y-values to equal eachother

now, here's where I think I begin to mess this up. I am trying to rearrange this into a quadratic equation.

a=-1 b=(1+k) c=(-k-1)

Then I use

now I'm stuck and don't know where to go from here.