Answer
$$940,800 \mathrm{N}$$
Work Step by Step
A horizontal strip at depth $y$ has length 6 and width
$$
\frac{\Delta y}{\sin 30^{\circ}}=2 \Delta y
$$
Thus,
\begin{align*}
F&=2 \rho g \int_{0}^{4} 6 y d y\\
&=6\rho g y^2\bigg|_{0}^{4}\\
&=96 \rho g
\end{align*}
If distances are in feet, then $\rho g=w=62.5 \mathrm{lb} / \mathrm{ft}^{3}$ and $F=6000 \mathrm{lb}$; if distances are in meters, then $\rho g=9800 \mathrm{N} / \mathrm{m}^{3}$ and $$F=940,800 \mathrm{N}$$
