Orthogonality & Least Squares · Hard problem

A least-squares problem is solved via $QR$ factorization: $R\mathbf x = Q^T\mathbf b$ with $R = \begin{pmatrix}1&2&3\\0&1&4\\0&0&2\end{pmatrix}$ and $Q^T\mathbf b = (3,4,12)$. Using back-substitution, find $x_1$ (the first entry of the least-squares solution $\mathbf x$).