a point on the coordinate plane is written as (x,y).. since you have two points on your picture think of it as the 1st (x,y) and the 2nd (x,y). When you use the formula and it says y2 - y1 that means take the 2nd y and subtract the 1st y.
When it says x2 - x1 that means take the 2nd x and subtract the 1st x.
It does NOT matter which one you pick for the 1st (x,y) or 2nd (x,y)
In your problem lets just say you decide (-6,-3) will be your 1st point, that mean -6 will be your x1 and -3 will be your y1
Then that means you have to make (1,6) your 2nd point, that means 1 will be your x2 and 6 will be your y2.
Now just substitute, y2 - y1 / x2 - x1 = 6 - -3 / 1 - -6 = 9/7
