Answer
$7.532\%$
Work Step by Step
Assume that the growth factor is $b$ the after 12 years, the growth factor would be $b^{12}$. Given that the investment decreased by $60\%$, the size of the original investment would be $1-0.6= 0.40$. Then
$$
\begin{aligned}
b^{12} & =0.40 \\
b & =0.40^{\frac{1}{12}} \approx 0.92648
\end{aligned}
$$
The effective annual rate by which the investment decreases per year is $1-0.92648=7.532\%$
