Consider the "journey" from (3, -1) to (8, 9).
The x-coordinate increases by 5, while the y-coordinate increases by 10.
Quick mental trial-and-error tells us that the two points lie on the line y = 2x - 7
Now we want the point of intersection of y = 2x - 7 and x - y - 2 = 0
Using substitution:
x - y - 2 = 0
x - (2x - 7) - 2 = 0
-x + 5 = 0
x = 5
y = 2(5) - 7
y = 3
The point of intersection is (5, 3).
The three points under consideration are (3, -1), (5, 3) and (8, 9).
Just dealing with x-coordinates: from 3 to 5 = 2 units, and from 5 to 8 = 3 units.
SOLUTION: the straight line x - y - 2 = 0 divides the line segment (3,-1) to (8,9) in the ratio 2:3