The problem goes; The entire figure is placed in the standard $(x,y)$ coordinate plane such that the vertices of the square are $A(6,6)$, $B(0,6)$, $C(0,0)$ and $D(6,0)$. The $x$ coordinate of $E$ is $3$. Which of the following is a line of symmetry for the figure ?
I was very sure that the answer is both $y=3$ and $x=3$, but to my surprise, the answer was only $x=3$. Why can't $y=3$ be the answer ?
(sorry for posting such an elementary question, but I'm not an expert)
The black line segment is all the possible points the $E$ could be.
As the picture tells, the line of symmetry will be $y = 3$ only when $E$ is point of intersection of black and red dotted line $(3,3)$.