1. The problem statement, all variables and given/known data http://img857.imageshack.us/img857/548/screenshot20120112at853.png [Broken] 3. The attempt at a solutionI reasoned that if U is a vector subspace, then the zero vector must certainly be an element of U. That is, [itex](0,0,0) \in U[/itex]. If this is true, then we can write for [itex]x_1 + x_2 + x_3 = a[/itex], [itex]0 + 0 + 0 = a = 0[/itex]. If a is a fixed value as is evident in the problem, then a cannot equal anything but zero. Does that sound about right?