The stock price for a certain corporation can be approximated for the years $2009$ through $2015$ by $f(x)=7\mathrm{.}4x^2-184x+1160,$ where $x=9$ corresponds to the year $2009.$ During what year did a local minimum stock price occur?

A local extremum occurs at a critical number of $f.$ Because the derivative exists for every $x,$ the only critical number$($s$)$ occur where the derivative is zero. Find the derivative of $f(x).$

$f^{\text{'}}(x)=$