There is a hemispherical shell on the ocean floor as shown in the figure below. Assume the pressure inside the shell is $1$PPa $($gage$).$ A mercury U$-$tube manometer is attached to the side of the shell $($as shown$)$ to indicate the seawater pressure outside the shell. What is the value of $h$ on the mercury manometer $($in $mm)?$ Clearly indicate your chosen datum. The specific gravity of the mercury is $13\mathrm{.}6$ and the specific gravity of the seawater is $1\mathrm{.}03$

$($Answer: $777mm-$ note: sometimes, I will give the answer for a problem to guide you towards completing it correctly, but I still expect to see full, clear work for the problem!$)$

Ocean surface

The answer is $777$mm but im not how to get to that answer