Answer
(a) $36$ inches
(b) $60$ inches
Work Step by Step
Step-1: Differentiate the following equation with respect to $l$,
$$S=6l^2$$
, we get,
$$\frac{dS}{dl}=12l$$
Step-2: When $l=3$ inches,
$$\frac{dS}{dt} = 12 \times 3 =36 inch^2 / inch$$ or, $36$ inches.
Step-3: When $l=5$ inches,
$$\frac{dS}{dt} = 12 \times 5 =60 inches$$
