The slope-intercept form is y=mx+b
Looking at the graph you know that the slope will be negative as the line goes down and the b will be where the line crosses the y axis
So b = -1
Find the slope using 2 points on the line and the formula of m=(y2-y1)/x2-x1)
Use point. (0,-1) and (1,-2). m=(-2-(-1))/(1-0)
m= -1
Now put into the slope-intercept form
y = -x-1