I know the answer for $"$a$"$ but still i would like someone to answer for botha and $"$b$".$

A salinity control gate, $5$ meters wide and hinged in the middle at a point $3$ meters above the bed, is utilised to regulate the mixture of fresh and saltwater, as illustrated in the figure below. The fresh water level is maintained at $6$ meters $($fixed$).$ As the saltwater level rises, the gate pivots to facilitate the mixing of water. When the salt water rises, the gate turns to allow mixing water.

a$.$ Determine the level of saltwater $(h)$ at which the gate is in balance. Consider the density of seawater at $20C$ as $1025k\frac{g}{m^3}$ and fresh water as $1000k\frac{g}{m^3}.$

b$.$ Identify the acceptable levels of saltwater $(h)$ at which the gate remains in equilibrium.

The density of seawater at $20C$ can be taken as $1025k\frac{g}{m^3}$ and the fresh water is $1000k\frac{g}{m^3}.$