## Applications of Integration: Force Due to Fluid Pressure

I'm working on a particular problem that involves the force on a face of a triangular prism. What I am wondering is if someone could explain how to find dA for the prism? If the prism is an equilateral triangle, then $dA=\sqrt3dy$ doesn't it? My book is showing that the area is $2\sqrt3ydy$ and I really cannot see how...