If there is a porpotional relationship then it there will be a pattern like, 7, 14, 21 etc. You can tell because the line will go straight in a perfect line and won't go at a different andle from the rest of the line.
6 - b/3a a = 10, b = 9
6 - 9/(3*10)
6 - 9/30
6 - 3/10
6 - 0.3
<span>= 5.7 or 57/10</span>

Point-slope form is
, where
is the slope and
is a point on the line.
Substitute the values. 
<em>Note: This is in point-slope form. Let me know if you need a different form. Also, if you have any more problems similar to this, I encourage you to try them on your own, and ask on here if you are having trouble.</em>