Expressing a vector in terms of the other two?

OK so I'm reviewing my midterm exam and my teacher pulled some answer out of nowhere. In the question I have three vectors [1, -1, 3], [-1, -1, 7], and [2, -1, 1] transposed. He wrote something down like insert X3=2 for the last vector but I have no idea how he got that. I just wanna know where do you start.