The equation can be written in the form f(x) = a(1 + r)x or f(x) = abx where b = 1 + r. but not sure if this is correct I need help making an excel formula
I can confirm that f(x)=a*(1+r)^x is an expression for exponential growth. f(x)=a*b^x is another form (used by Excel's LOGEST() function, if you decide to use LOGEST() to perform any of the regressions: https://support.microsoft.com/en-us/...rs=en-us&ad=us ). The form I think most learn in school (especially when you get into calculus) is f(x)=a*exp(c*x), because base e exponential/logarithms are especially "natural" (ha-ha) when doing calculus. If your sources are saying that f(x)=a*(1+r)^x is the "correct" form, then that is probably the form you will want to derive. If you are going to be doing regressions in Excel, it will probably be easiest to use the f(x)=a*b^x (equivalent to ln(f(x))=ln(a)+x*ln(b) which is the form used by the linear regression algorithms) form, since that is the form used by the LOGEST() function.

From there, we might need to better understand exactly what help you need in order to make an Excel formula.