The point slope form of a line is:
(y-y1)=m(x-x1) ............(1)
where
m=slope = -3 for this problem.
(x1,y1) is a point contained in the line, which equals (1,2) for this problem.
Substitute m, x1, y1 into equation (1) to get the equation of the line in point slope form.
