Orthogonality & Least Squares · Medium problem

A spring's stretch is believed to be proportional to the applied force, stretch $= k\cdot\text{force}$. Three trials give (force, stretch) $= (1,3), (2,5), (4,9)$ in consistent units. Using the least-squares value of $k$ (minimizing the sum of squared errors over all three trials), predict the stretch at a force of $9$.