Whats the completr answer pls

$16\mathrm{.}15$ A cut slope was excavated in a saturated clay. The slope angle, $,$ is equal to $40$ with the horizontal. Slope failure occurred when the cut reached a depth of $8\mathrm{.}5$ m $.$ Previous soil explorations showed that a rock layer was located at a depth of $12$ m below the ground surface. Assume an undrained condition and that $=18\mathrm{.}5k\frac{N}{m^3}.$

a$.$ Determine the undrained cohesion of the clay $($Figure $16\mathrm{.}13).$

b$.$ What was the nature of the critical circle?

c$.$ With reference to the top of the slope, at what distance did the surface of sliding intersect the bottom of the excavation?