Coefficient Correlation r of Exponential Functions Regression

Question

I'm writing an exponent regression calculator $Ae^{Bx}$ Sample Data Set (X,Y) is

(9, 1)
(7, 10)
(6,11)
(20, 10)
(15, 1)

A = 5.287 and B = -0.0232. So $F(x) = 5.287e^{-0.0232x}$ which is fine.

However, what is the right formula to calculate Coefficients Correlation r ?

1. $r = \sqrt{R2} = \sqrt{1 - \frac {RSS}{TSS}}$ which can not be applied because R2 = -0.275 Reference Here

2. $r = \sqrt{\frac {ESS}{TSS}} = 0.311$ Reference Here

3. $r = \frac {\sum{(X-\bar X)(Y-\bar Y)}}{\sqrt{\sum{(X-\bar X)^{2}} \sum{(Y-\bar Y)^{2}}}} = -0.00109$

4. On the other hand, My Casio FX570 give $r = -1.0782$ (with the same A & B)

Which one is correct and what is the formula that Casio use ?