I think it's helpful to draw a diagram of the problem. Plot the two points A and B on a coordinate grid and connect them with a line segment. Then make that line segment the hypotenuse of a right triangle so that the horizontal leg goes to the right from point A and then the vertical leg goes down from B and they meet at a right angle.
If you calculate x2 - x1 you get that the length of the horizontal leg is 15.
If you calculate y2 - y1 you get that the length of the vertical leg is 12.
If we want to find the point that is 3/10 of the way from A to B, we can take 3/10 of each of these lengths and then add those numbers to point A's coordinates.
15(3/10) = 4.5 and 12(3/10) = 3.6
So our point's coordinates would be (-4 + 4.5 , -8 + 3.6) = (0.5, -4.4)
