The difference between the x coords of A and B is 5-(-4)=9. That between the y coords is 1-(-5)=6.
The y coordinate of C is ⅔ the way from A to B, that is, ⅔×6=4 units "north" of A, which is -5+4=-1.
The x coordinate of C is ⅔ the way from A to B, that is, ⅔×9=6 units "west" of A, which is 5-6=-1.
C is the point (-1,-1).