You get this by looking for the rise over run. using ghe two points located near the origin (0, 0), you can count that the line goes two units down and two units to the right. that means the line "rises" by -2 (though we can see it actually falls) and it goes over 2, towards the positive values of x. that means the rise over run is (-2/2), or -1. the slope of the line is negative one, because you can go down one unit, right one, down one, right one, over and over to create the line.
Since AB and DC are parallel, then the alternate interior angles are congruent because there is transversal cutting the parallel lines (BC). So m<B is supplementary to m<C
