\begin{align*} P(\mu - 2\sigma \le X \le \mu + 2\sigma) & = P(15 - 2 \sqrt{10.5} \le X \le 15 + 2 \sqrt{10.5})\\ & P( X \in \{9, 10, 11, ... , 19, 20, 21 \} ) \end{align*}