Please provide a detail solution; the correct answer is $90\mathrm{.}0$ kn$/$m$^2.$

A $2$ m wide surface supported strip footing will be used to support a masonry bearing wall for a commercial building. The wall loading imposed onto the foundation will be $300$ kN per meter of wall length. The soil under the foundation has a saturated unit weight of $20k\frac{N}{m^3}.$ If the water table is at the surface, what is the effective vertical stress at $3$ m below the centre of the foundation when the foundation is loaded? $($use the attached information$)$

$=\frac{q}{}\{+sincos(+2)\}$