First, draw a graph of y= 4- x^2 (NOT "x2", please) and imagine rotating that around the x- axis. You should be able to see that any point on the graph goes in a circle around the x-axis and the radius of that circle is the y coordinate of the point: radius r= y= 1- x^2. That circle has area r^2= (1- x^2). You can think of that as a very thin disk with thickness " " and so volume (1- x^2) . You find the volume of the entire thing by adding up all of those disks: (1- x^2) . That is what you have learned as a "Riemann sum" and, in the limit, becomes the integral (1- x^2) dx.
Can you do that integral?