Using rise/run you can count the units the line moves from one point to another, from the left point to the right point the line travels down 3 and over 10 so you get -3 as your rise (it is negative because the line is traveling downwards) and 10 as your run, so your slop is -3/10. By looking at the graph you can see that your y-intercept is 1/2. SO when plugged into y=mx+b with m as -3/10 and b as 1/2 you get the equation y=-3/10x+1/2, or letter D.
