$5-$ The doubly symmetric, built$-$up box$-$beam $\backslash ($ A B C $\backslash )$ is constructed from four timber planks that are nailed together as shown $($all dimensions are in mm $).$ The beam is safely loaded as shown. a$)$ Determine the maximum shear stress in the timber. b$)$ If the shear capacity of a single nail is $5$ kN $,$ determine the longitudinal nail spacing, $\backslash ($ s $\backslash ),$ required to achieve a factor of safety $($FS$)$ of $4\mathrm{.}0.$ Round your answer in the safe direction to the nearest $10$ mm $.[2$ Marks$]$