I think I got somewhat of an answer for this, but need to confirm.
Ok, In the case of the shell method, and being revolved around the y axis, then plug in the given x value for the line (x = 4) into the given y equation. That gives you one limit of integration, you can assume the other limit of integration is 0. So in the 1st problem the limits of integration are
In the case of the shell method, revolving around the x axis, you would set "x = something" equal to the line equation of "x = 4", in other words, something = 4, to find the y value. In the case of problem two. You would have
So you can say the limits of integration are 2 and 0 (the other limit of integration is assumed to be 0)