Its coordinates with respect to the given basis are \begin{equation} (a_0,a_1,\dots,a_n) \end{equation} because it's expressed as a linear combination of the elements in the basis with coefficients $a_0,\dots,a_n$.
Its coordinates with respect to the given basis are \begin{equation} (a_0,a_1,\dots,a_n) \end{equation} because it's expressed as a linear combination of the elements in the basis with coefficients $a_0,\dots,a_n$.