Find the orthogonal projection of u = (5, 6, 7, 2) on the solution space of the homogeneous linear system:

$\displaystyle x_1 + x_2 + x_3 = 0$

$\displaystyle 2x_2 + x_3 + x_4 = 0 $

If I knew how to set this one up, I could run with it, but I'm a little confused. I know it will eventually involve the normal system $\displaystyle A^TAx = A^Tb$, but I'm not sure how to find A and b with a question worded this way.