A cylindrical water tank with radius 2m is installed in such a way that the axis is horizontal and the circular cross sections are vertical. Water is put into the tank so that the depth of the water is 3m. What percentage of the total capacity of the tank is being used? Round off to the nearest percentage point.

I have tried computing the integral of V= (2pix(4-x^2)^.5)dx where I got -(1/3)cos^3x ... I am unsure as to what else I have to do. I am completely lost on this problem.