Orthogonality & Least Squares · Medium problem

A chemist models a reaction rate $R = k_1[A] + k_2[B]$ (no constant term). Three trials give $([A],[B],R) = (1,1,4),\ (2,0,5),\ (3,-1,9)$. Using the least-squares estimates $k_1,k_2$, find $k_1+k_2$.