How do you write eighty seven billion, three hundred million, eleven

The function can be considered to be a horizontal translation and a vertical reflection of a function that decays by a factor of 5/20 = 1/4 as x increases by 2.
.. y = -20(1/4)^((x +2)/2)
This can be simlified to
.. y = -5*(1/2)^x

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If you allow vertical translation, any number of growth or decay functions can be written that will have these two points on their curve. The graph shows a growth function that passes through the same two points.