while you can use shell method, i think it's easier to rewrite y=x as a function of y and use disc method to rotate about the y-axis.
as a function of y, y=x becomes x=y.
using disc method, the volume of the solid is:
a and b are the upper and lower bounds of integration. in this problem,
r(y) is the distance between the function and the axis of rotation. in this problem,
plugging in: volume=