\frac{\partial L(w,b)}{\partial w} = 2X^\top(Xw+b-y) = \begin{bmatrix}1 & 3 \\ 2 & 4\end{bmatrix}\begin{bmatrix}10 \\ 20\end{bmatrix} = \begin{bmatrix}70 \\ 100\end{bmatrix}
\frac{\partial L(w,b)}{\partial b} = 2\begin{bmatrix}1 & 1\end{bmatrix}(Xw+b-y) = \begin{bmatrix}1 & 1\end{bmatrix}\begin{bmatrix}10 \\ 20\end{bmatrix} = 30