Step $1$

Recall that a function $fis$ continuous $at$ a number $aif$ the following holds, where $f(a)aisin$ the domain $off\underset{xa}{lim}f(x).$

$\underset{xa}{lim}f(x)=f(a)$

Note that the function $f(x)=x\sqrt[\phantom{2}]{41-x^2}$ has the following domain and $is$ continuous $on$ this domain.

$-\sqrt[\phantom{2}]{?},x\sqrt[\phantom{2}]{?}$

Therefore, $a=4---$Select$---in$ the domain $off,sof(a)---$Select$---$ defined.