A long wire situated on the positive x-axis, with the left at the origin has density δ{x) = x3 (kg/m) at a point x ≥ 0. How long should the wire he cut so that its mass is M kg? In other words, what is L (in terms of M) so that the mass of wire between the left end 0 and the right, end L, where we make the cut, M?