Hello, so I've been having lots of trouble with a math problem. I can differentiate functions but i'm kinda confused on what to do when it is like y=4 So say you have the given: y= x^2 and then y=4, x=0, x=2. Rotate about the y-axis. I'm know you have to get it into respect to y so x=sqrt(y) but what would I do from there?! you can't put x in terms of y for y=4! i'm confused. Can somebody help me out or point me in the right direction.