When trying to find the point-slope equation of a line you need to pick a point and plug it into the plain equation with your slope.
-3 = 1/3(-1) +b
Multiply your slope with your x
-3 = -1/3 +b
Move the -1/3 to the other side by doing the opposite operation, in this case being addition.
-3 +1/3 = b
-2 2/3 = b
y = 1/3x - 2 2/3