1. Given $a \geqslant-2$, and $A=\{x \mid-2 \leqslant x \leqslant a\}, B=\{y \mid y=2 x+3, x \in A\}, C=\left\{t \mid t=x^{2}, x \in A\right\}$, if $C \subseteq B$, then the range of values for $a$ is $\qquad$ .
$B=[-1,2 a+3]$, if $a \in[-2,0] \Rightarrow C=\left[a^{2}, 4\right] \Rightarrow 2 a+3 \geqslant 4$, contradiction;
if $a \in(0,2] \Rightarrow C=[0,4] \Rightarrow 2 a+3 \geqslant 4 \Rightarrow a \in\left[\frac{1}{2}, 2\right]$;
if $a>2 \Rightarrow C=\left[0, a^{2}\right] \Rightarrow 2 a+3 \geqslant a^{2} \Rightarrow a \in(2,3]$. In summary, the range of $a$ is $\left[\frac{1}{2}, 3\right]$.