The function R$($x$)$ billion dollars models revenue from new$-$car sales for franchised new$-$car dealerships in the United States when x million dollars are spent on associated advertising expenditures, data from $1\mathrm{.}2$ x $6\mathrm{.}5.$

$($b$)$ Use the linearization model to estimate the revenue when $$1\mathrm{.}6$ million is spent on advertising.

$($c$)$ Use the linearization model to estimate the revenue when $$2$ million is spent on advertising.