[tex]\it x \circy=xy+4x+4y+12 = xy+4x+4y+16-4 =
\\ \\
= (xy+4y)+(4x+16)-4 = y(x+4) +4(x+4)-4 =(x+4)(y+4)-4
[/tex]
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[tex]\it x\circ x =12 \Rightarrow (x+4)(x+4) -4=12 \Rightarrow (x+4)^2=16 \Rightarrow
\\ \\
\Rightarrow x+4=\pm4 \Rightarrow x+4 \in\{-4,\ 4\}|_{-4} \Rightarrow x \in\{-8,\ 0 \}[/tex]