1. Where is the attachment, can't see it.
2. 5 + 4 + 6 = 15 balls in total. 6 are white so its 6/15=2/5, ie the probability is 2/5.
3. Given a line with two points (A,B) and (C,D), the midpoint , (E,F), is found as E=(A+C)/2 and F=(B+D)/2.
So, in this example we have
A=5, B=-1, C=x, D=2, E=F=0
Using basic algebra we find A=-C (subbing in E=0).
A=5 thus C=-5
Wait... If you apply the same reasoning we find that B=-D, which is false so I think you may have given some incorrect numbers. If your point A and point B co-ordinates are correct then it is your midpoint that is incorrect.