# Volumes by Slicing: why not y-axis?

Find the volume of the solid generated by revolving the region bounded by the graphs of y = x^2 and y = 4x − x about the line y = 6. We should calculate it according to this integral ∫[(6-x^2)^2-(6-(4x-x^2))^2] dx. Why do we integrate it for x-axis not y-axis?