Answer
$x=\pm\sqrt{11}$
Work Step by Step
$f(x)=x^{2}$
$f(x)=11$
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$x^{2}=11$
According to the principle of square roots:
For any real number $k$ and any algebraic expression $X$ :
$\text{If }X^{2}=k,\text{ then }X=\sqrt{k}\text{ or }X=-\sqrt{k}$
$x=\pm\sqrt{11}$