\(X _{ cm }=\frac{\int xdm }{\int dm _{\ell}}\)
\(=\frac{\lambda_{0} \int_{0}^{\ell}\left( x -\frac{ x ^{3}}{\ell^{2}}\right) dx }{\int_{0}^{\ell} \lambda_{0}\left(1-\frac{ x ^{2}}{\ell^{2}}\right) dx }=\frac{\frac{\ell^{2}}{2}-\frac{\ell^{4}}{4 \ell^{2}}}{\ell-\frac{\ell^{3}}{3 \ell^{2}}}=\frac{3 \ell}{8}\)