a.    Effective Stress  P or σ_v @ -18m  = (18.8 kN/m³ - 9.81 kN/m³) 12m = 107.9kN/m² = 107.9kPa

b.    Total Stress  P or σ_v  @ -18m = 107.9kPa + γ_w (18m) = 107.9kPa + 9.81 kN/m³ x (18m) =  284.5kPa