For the truss shown above, determine:

$($a$)$ a mathematical expression relating the tension in the cables $($T$1$ and T$2)$ to the applied force P $=2\mathrm{.}2$E$6$ N shown $($FBD useful here$)$

$($b$)$ the displacement of joint D when the load is applied $($problem statement shows unloaded truss geometry$)$

$($c$)$ the stress in member BD