A helical compression spring with squared and ground ends is made of a $2\mathrm{.}7$ mm steel wire with $5\mathrm{.}05$ turns of the active spring. The operation load of the spring can be considered static. If the spring wire has a spring constant of $9$ N$/$mm$,$ and the "solid" force of the spring Fs $=381$ N$,$ determine the free length $($natural length$)$ of the spring. $[$mm$]$