Using the Coulomb analysis approach, determine the active force on the retaining wall shown below. $Q$ is a line load $($out of plane$)$ with $Q=10**xk\frac{N}{m}($where $x$ is the last digit of your student ID$).$ Express $P_a$ as a function of $,$ and obtain the critical $$ and the corresponding maximum $P_a$ by trial and error of $$ values from $40$ to $70.$