I'm having some trouble on all of these problems. For the first one, I don't know how to do it because of the two variables (although they're both the same). For #2, I figure to use the disk method and set the integral from 5 to 13. The only problem is when I substitute 5 and 13 in for x and subtract them after I've integrated, I come up with a negative number. For #3 and 4 the problem I have is that I'm trying to integrate on the interval of x=0 to x=9. Where x=9 however, the formula comes out to 0, so the volume does as well. Once I get #3, #4 should be cake. Any guidance is much appreciated.