Find $f^{\text{'}}(x)$ and approximate $($to four decimal places$)$ the value$($s$)$ of $x$ where the graph of $f$ has a horizontal tangent line.

$f(x)=0\mathrm{.}20x^4-0\mathrm{.}6x^3-6\mathrm{.}8x^{2}19\mathrm{.}0x5$

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