How do you write an equation for (-5,-1), (0,-1)

Both the points have a coordinate where y is -1, so the equation is simply -1

y=-1

however if you wanted to really show the working out, it looks like this:

y=mx+c

m=change in y/change in x

m=(-1-1)/(-5+0)=0

subsitute one of the coordinates into y=mx+c

-1=0+c

c=-1

so then y=-1