Q$3$: answer only One :A $7\mathrm{.}0-$m$-$high retaining wall shown in Fig. $3.$ retains a granular backfill where ${}^{\text{'}}=30,=20,$ and $=18\mathrm{.}5k\frac{N}{m^3}.$ a$.$ determine the location, direction, and magnitude of the active thrust $Pa$ using Rankine's equation. b$.$ Determine the active thrust Pa on the wall using Coulomb's method. C$.$ if the backfill is caly with $c=35$kpa,${}^{\text{'}}=30,=20,$ and $=18\mathrm{.}5k\frac{N}{m^3}$ find $Pa?[10$M$]$

where

$K_{}=(\frac{cos\frac{cos-\sqrt[\phantom{2}]{co{s}^2-co{s}^2{}^{\text{'}}}}{cos+\sqrt[\phantom{2}]{co{s}^2-co{s}^2{}^{\text{'}}}}}{\sqrt[\phantom{2}]{sin(-)}+\sqrt[\phantom{2}]{\frac{sin({}^{\text{'}}+)sin({}^{\text{'}}-)}{sin(+)}}}{)}^2$

Fig$3$

Good Luck