given that =mi+9j, =6i+6j and =12i+4j,find the value of m if point P,Q and R are collinear.
In order for P Q and R to be colinear they must lie on the same line. This is represented by the dashed line there. We already know the y coordinate of the missing vector. This is shown by the solid white line. So the x value must be somewhere on that line AND the dashed line. That's your answer.