use 2 points to set up the equation of a straight line and test afterwards if the 3rd point is located on the line:

. If R(5, 6) lies on the line the coordinates of R must satuisfy the equation of the line:

. That means from

and from

there doesn't exist one value for r so that the point R is "produced" by the equation. Thus

I'll leave the next problem for you.

(For confirmation only: F is placed on DE)