$(2)$ Solution to the quadratic equation $a{x}^2+bx+c=0$ is:

QUESTION $2(25$ marks$)$

An existing mass concrete wall is retaining and embedded in Silty Sand, and founded on Gravel. The assumed site conditions as shown in Figure $2$a shall be adopted in design analysis. The Gravel layer, which is thin, highly permeable and connected to an aquifer, has a constant water pressure of $60$ kPa $.$ The level of the groundwater table within the Silty Sand layer on both sides of the wall stays at $25$ mPD $.$ The relevant material and design parameters are as follows:

$\backslash$table$[[$Water$,$Density, ${}_w=10k\frac{N}{m^3}$