All this is doing is finding the distance between the y-coordinates and dividing it by the distance between the x-coordinates. I marked these two as points 1 and 2 above, so you can plug in all 4 values and solve:
so hmmm then we know the slope of that line is -2/3, so we're really looking for the point-slope form of a line with a slope of -2/3 and that passes through (-3 , 8)