$($b$)$ A timber beam is freely supported on supports $6m$ apart. It carries a uniformly distributed load of $12k\frac{N}{m}$ and a concentrated load of $9kN$ at $2\mathrm{.}5m$ from the left support. If the stress in timber is not to exceed $8\frac{N}{m}{m}^2,$ design a suitable section making the depth twice the width $($Fig$.5).$