First choose two points, if you can choose two points that lie on a grid intersection, we'll use (0,4) and (3,3)
Using the formula y2-y1 / x2-x1 determine the slope so plug in the numbers
3 - 4 / 3 - 0 which equals -1 / 3 remember RISE over RUN so the slope is for every 3 to the right its 1 down because it's a negative.
You could also just look at the picture determine the slope