SOLUTION: The midpoint of the line segment ST has coordinates (3,6). If S is the point (4,1), find the coordinates of T. I've asked this question before and received an answer from a tuto

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Question 715154: The midpoint of the line segment ST has coordinates (3,6). If S is the point (4,1), find the coordinates of T.
I've asked this question before and received an answer from a tutor although the answer in my textbook said that he is incorrect. Also, I've received the same answer as him so I'm quiet confused as to what I'm do do right now.
Here's my working out. Please tell me where I went wrong.
Let the coordinate of T be (x,y)
Then by the midpoint formula ((x+3)/2, (y+6)/2)=(4,1)
Midpoint formula for x.
(x+3)/2=4
(x+3) = (I've multiplied both sides by 2 though I've forgotten why so if it's possible, please do explain why )
x = 5
-------
Midpoint formula for y.
(y+6)/2 = 1
y+6 = 2
y = -4
But the answer in my book says that the coordinate is (2,11). Why is that?

Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
The midpoint of the line segment ST has coordinates (3,6). If S is the point (4,1), find the coordinates of T.
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S:(4,1)
M:(3,6)
T:(x,y)
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Mx = (4+x)/2 = 3
Mx = 2
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My = (1+y)/2 = 6
Mx = 11
M is (2,11)