We know that the point C lies in the segment AB. Since the ratio between AC and CB is 3:1, we also know it lives in the first 1/4 of the segment.
To calculate the point at 1/4 of the segment, we can find the midpoint, and then the midpoint of the segment made with point A and the midpoint.
The midpoint y-coordinate is given by:
Now, calculating the midpoint between the midpoint of the segment AC and A, we have:
And this is our answer. The y coordinate of the point C is 5.5