Question 1049156
G is some unknown endpoint,  G(x,y).


Midpoint for GN will need to fit  {{{(7+x)/2=9}}}, and {{{(5+y)/2=11}}}.


Solve for x and y.


7+x=18
x=18-7
x=11
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y+5=22
y=22-5
y=17
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Correct way to get the point G(11,17) but did not uncover the friend's mistake.


The only common trouble I can see is that the coordinates  (15,13) are each four off but in different directions.  My only guess is that friend mistakenly added or subtracted 4 to each x & y result.