#### Problem 65E

Biologists have proposed a cubic polynomial to model the length $L$ of Alaskan rockfish at age $A:$

$$L=0.0155 A^{3}-0.372 A^{2}+3.95 A+1.21$$

where $L$ is measured in inches and $A$ in years. Calculate

$$\left.\frac{d L}{d A}\right|_{A=12}$$