#### Answer

$\pm12$

#### Work Step by Step

$\pm\sqrt {144}$
According to the textbook, a number, $a$, is a square root of another number, $b$, if $a^2=b$. Since $12^2=144$, $12$ is a square root of $144$.
Given this, we can deduce that: $\pm\sqrt {144}=\pm12$