What is the slope of the line passing through the points (0, −5) and (4, 2) ?
It would be a good idea for you to watch and try and understand this video :)
https://www.khanacademy.org/math/algebra/two-var-linear-equations/slope/v/slope-of-a-line-2
or possibly this might be easier to understand
http://www.teacherschoice.com.au/Maths_Library/Gradient/gradient_-_two_fixed_points.htm