where m is the slope of the line, and y_1 and x_1 are the y and x coordinates of any point on the line. You have two known points, A and B, so you can use either of these for the x_1, y_1 values.
Since you have two points, you can calculate the slope. The slope of a line is the difference in Y coordinates of two points divided by the difference in X coordinates of two points. Lets call point 1 A and point 2 B, the slope equation is