Question $4(10$ points$)$

A freshwater marsh is drained to the ocean through an automatic tide gate that is $4$ ft wide and $3$ ft high. The gate is held by hinges located along its top edge at A and bears on a sill at B $.$ If the water level in the marsh is $\backslash (\backslash$mathrm$\{$h$\}=6\backslash$mathrm$\{$ft$\}\backslash ),$ determine the ocean level d $($in ft$)$ for which the gate will open. $($Specific weight of salt water $\backslash (=68\backslash$mathrm$\{$lb$\}/\backslash$mathrm$\{$ft$3\}\backslash ),$ Specific weight of fresh water $\backslash (=62\mathrm{.}4\backslash$mathrm$\{$lb$\}/\backslash$mathrm$\{$ft$\}3\backslash ))$