The slope of the line between the points (a,b) and (c,d) is equal to $\displaystyle\frac{{{b}-{d}}}{{{a}-{c}}}\usepackage{cmbright}\Huge\$ (which is also equal to $\displaystyle\frac{{{d}-{b}}}{{{c}-{a}}}\usepackage{cmbright}\Huge\$ ). Be sure to use the coordinates in the same "order" in the numerator and denominator. That is, if the numerator is y-coordinate of first point minus y-coordinate of second point, then the denominator should be the x-coordinate of the first point minus the x-coordinate of the second point.