Let $f(x)$ be a differentiable function with $f(2)=6$ and $f^{\text{'}}(2)=3.$

$($a$)$ The equation of the line tangent to the graph of $f$ at $x=2$ is

$$

$($b$)$ Using the tangent line to the function $f$ at $x=2$ to approximate the value of $f(2\mathrm{.}1).$ we obtain $f(2\mathrm{.}1)$~~ $$ is approximately equal tol.