$I{b}_f$ exerted on it by the water upstream?

$21.$ Force on $V-$shaped dam $-M.$ A vertical dam has the shape of a $V$ that is $3m$ high and $2m$ wide at the top, which is just level with the surface of the water upstream of the dam. Use two different methods to determine the total force $(N)$ exerted by the water on the dam.