Answer
$f(x)=\pm\sqrt{19}$
Work Step by Step
$f(x)=19$
According to the principle of square roots:
For any real number $k$ and any algebraic expression $X$ :
$\text{If }X^{2}=k,\text{ then }X=\sqrt{k}\text{ or }X=-\sqrt{k}$
$f(x)=\pm\sqrt{19}$