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Consider the function $y = f(x)$ graphed below.

graph of an decreasing, concave down function passing through the points A and B, with A to the left of B.

The two points marked on the graph are $A = (-1,{-1\over4})$ and $B = (2,-16)$ . Give a possible formula, of the form $Ca^x$ , for the function graphed.
$y =$