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 that the difference of y-coordinates is in the numerator and the difference of the x-coordinates is in the denominator.