From looking at the line you can already tell that the slope is going to be negative, and the y-intercept is going to be 9, since that's where y is when x is 0. You can see that when x is one, y is 7. When x is 2, y is 5. And when x is 3, y is 3.
So:
0: 9
1:7
2:5
3:3
And the difference of y over the difference of x is the slope