so ask yourself, "How far did i rise from the first point to the second point(the 'y' values)?" answer....from 2 to 5 takes 3 positive steps so...3 is the "rise" .
now, do the same with the "run".
ask yourself, "How far did i run from the first point to the second point?" answer....from 1 to 2 takes 1 positive step so...1 is the "run" . This gives a slope of 3/1.
Next, you have to put this slope and one of the points into that "point-slope" formula.
y-y1 =m(x-x1 ) ----> we will use the point (1,2) = (x1,y1) here
y-2=3(x-1)=3x-3
add 2 to both sides and you get your equation...
y = 3x-1