7. The function $f(x)=a x^{2}+b x+3 a+b$ is an even function, with the domain $[a-1,2 a](a$ 、 $b \in \mathbf{R})$, its range is $\qquad$ .
7. $\left[1, \frac{31}{27}\right]$.
$a-1+2 a=0$, then $a=\frac{1}{3}$. Also, $f(x)$ is an even function, so $b=0, f(x)=\frac{1}{3} x^{2}+1, x \in$ $\left[-\frac{2}{3}, \frac{2}{3}\right]$