In Exercises 9-17, W is a subset of R^3 consisting of vectors of the form x = [x1; x2; x3]. In each case, determine whether W is a subspace of R^3. If W is a subspace, then give a geometric description of W.
9. W = {x: x3 = 2x1 - x2}
10. W = {x: x2 = x3 + x1}
11. W = {x: x1x2 = x3}
12. W = {x: x1 = 2x3}
13. W = {x: x1^2 = x1 + x2}
14. W = {x: x2 = 0}
15. W = {x: x1 = 2x3, x2 = -x3}
16. W = {x: x3 = x2 = 2x1}
17. W = {x: x2 = x3 = 0}
18. Let a be a fixed vector in R^3, and define W to be the...