I believe the answer is 3 3/4.
You're original equation has a -81, so the factored form will have to have a positive and negative multiplying one another to achieve that...
f(x) = x(x + 9)(x - 9)
We have the function
and we want to find a function that has the same y-intercept than the previous function.
First, let's find the y-intercept by subtituting 0 for 'x'.

Now that we found that y-intercept =-3, any lineal function of the type:
will have the same y-intercept. Where 'a' can take all the real values.
Also, any quadratic function of the type:
will have the same y-intercept. Where 'a' and 'b' can take all the real values.
Y=1000*1.03^x is the correct compound recursive formula!