Consider the following function.

f$(3)=18,$ f$\text{'}(3)=3\mathrm{.}6$; x $=3\mathrm{.}5$

$($a$)$ Write a linearization for f with respect to x$.$

fL$($x$)=$

$($b$)$ Use the linearization to estimate f at the given input.

fL$(3\mathrm{.}5)=$