11. Given the function $y=-\frac{x-a}{x-a-1}$, the graph of its inverse function is centrally symmetric about the point $(-1,3)$, $a=$ . $\qquad$
11. $a=2$.
$y=-\frac{x-a}{x-a-1}=-1-\frac{1}{x-a-1}$, then $(x-a-1)(y+1)=-1$. It is symmetric about the point $(a+1,-1)$, so $a+1=3, a=2$.