First, we know that it's a line so its equation must be y=ax + b in which a represents the slope, b the origin, x and y the coordinates of a point.
We know that the slope is 2 (so a = 2) and that this line passes through the point A (3, -1) so x = 3 and y = -1. We need to find b.
y = ax + b
-1 = 2*3 + b
-1 = 6 + b
b = -7.
So the equation of this line is: y = 2x - 7