Orthogonality & Least Squares · Medium problem

A chemist estimates an unknown concentration $x$ from three instrument readings, each obeying (sensitivity)$\cdot x =$ (reading): sensitivities $(1,1,2)$ give readings $(3,2,5)$. The instruments disagree, so $x$ is estimated by least squares. Find the resulting sum of the squared instrument errors (the residual sum of squares).