The pressure of a liquid at a height is given by: 
P=ρgh
Where, ρ = density of the liquid
h = depth if the point from the surface of the water.

Given,
ρ=1000kgm⁻³
h=6m
$\Rightarrow$P=1000×6×10Nm⁻²
Area=8×10=80m²
Now, the thrust at the bottom of the tank can be mathematically defined as the product of the pressure of the tank and the area of the tank. 
Thus we can write:
Thrust (F)=Pressure ×Area=1000×6×10×80 N.