Question

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Answer 1

Let
\[\Large M(x_M,y_M)\]
be the center !
Then, we can say that :
\[\Large x_M=\frac{-1+1}{2}=0~~,~~y_M=\frac{1+(-5)}{2}=-2.\]
So the center is the point :
\[\Large M(0,-2).\]

Answer 2

Where does -1+1 come from?

Answer 3

The vertices of that graph are :
(0,1) (0,-5) (1,-2) (-1,-2).