Let me first check on your first answer.
AB is divided in a ratio of 3:2.
3 + 2 = 5, so point C is 3/5 of the way from A to B and 2/5 of the way from B to A.
Work on x-coordinate of point B.
3.6 - (-6) = 9.6
3/5 * 9.6 = 3.2
5 * 3.2 = 16
16 + (-6) = 10
x-coordinate of A is 10
Work on y-coordinate of point B.
5 - (-0.4) = 5.4
5.4/3 = 1.8
1.8 * 5 = 9
5 - 9 = -4
Good job!
Now onto point D.
D is in a 4:5 ratio from C to B.
4 + 9 = 9, so C is 4/9 of the way from C to B.
Work on x-coordinate of D
10 - 3.6 = 6.4
6.4/9 = 6.4/9
3.6 + 4 * 6.4/9 = 32.4/9 + 25.6/9 = 58/9
Work on y-coordinate of D.
-4 - (-0.4) = 3.6
3.6/9 = 0.4
-0.4 - 4 * 0.4 = -0.4 - 1.6 = -2
D(58/9, -2)