A = (-1, 1)
B = (-3, 3)
slope = 3 - 1 / -3 - (-1)
slope = 2 / -2
slope = -1
Lets apply the point slope line equation:
y - yo = m(x - xo)
where
(xo, yo) is a point that lies in the line
m = slope
We can use any of the two points to calculate the equation. I'm going to choose (-1, 1). We have:
y - yo = m(x - xo)
y - 1 = -1(x - (-1))
y - 1 = -1(x + 1)
y - 1 = -x - 1
y = -x -1 + 1
y = -x
or
-x + y = 0
Answer: the equation of that line is y = -x or -x + y = 0