a $6$ m high retaining wall is to support a soil with unit weight $=17\mathrm{.}4$ kn$/$m$3,$soil friction angle $=26$o $,$ and cohesion c$=5$ kn$/$m$2.$ determine the rankine active force per unit length of the wall both before and after the tensile crack occurs, and determine the line of action of the resultant in both cases.