Usually which depends which method is easier to take the integral of one could produce a rather difficult integral and one can provide a much simpler one, when doing these volume of revolution problems you partition the area by sections that are perpendicular to the axis of revolution you cut section that are perpendicular to the x-axis in your case so you will have a thickness of dx, because your partition are happening are happening when values of x change in this case. so what that means you need to get your function in terms of x which they usually are in a manner like this