In 9 - 12 determine whether the subset W = \{f(a,b) : a, b \in \mathbb{R}\} is a subspace of \mathbb{R}_2[x] for the given f(a, b).
9. f(a,b) = (a - b) + (2a + b)X + (3a + 4b)X^2.
10. f(a, b) = (2a - 3b + 1) + (-2a + 5b)X + (2a + b)X^2.
11. f(a, b) = ab + (a - b)X + (a + b)X^2.
12. f(a,b) = (b + 1) + (a + 2b + 1)X + (2a + 3b + 1)X^2.