Answer
$\frac{dL}{dA}\Big |_{A=12}=1.718$
This means at age 12, the length of the Alaskan rockfish is increasing at a rate of 1.718 inches per year.
Work Step by Step
$\frac{dL}{dA}=0.0465A^2-0.744A+3.95$
When $A=12$, $\frac{dL}{dA}=0.0465(12)^2-0.744(12)+3.95=6.696-8.928+3.95=1.718$
This means at age 12, the length of the Alaskan rockfish is increasing at a rate of $1.718$ inches per year.
