Your answer is n=2. thank me if this helped you. :)
Since you don't provide the coordinates of the point W, I will help you in a general form anyway. In the Figure below is represented the segment that matches this problem. We have two endpoints U and V. So, by using the midpoint formula we may solve this problem:

Therefore:

So we know
but we also must know 
Finally, knowing the points U and W we can find the endpoint V.
-1 / 7 = x
You need to combine the 2x and 5x then devise the 7x from -1