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graph of an decreasing, concave down function passing through the points A and B (A to the left of B) and having a horizontal asymptote as x goes to large values, graphed as a red line.

Give a formula for the function illustrated using a vertical shift of an exponential function. The two points marked on the graph are $A = (-1,9)$ and $B = (1,3)$ . The red horizontal line is given by $y=1$ , and is a horizontal asymptote of the function.

$y=$