Can you show it's a subspace?

Assuming that you can, if you want to show that the dimension is at least 2, consider the following two cases :

(1)and are linearly independent : then the subspace contains the span of , which is two dimensional (in fact, try to show that it'sequalto the span of );

(2)are linearly dependent : then the space is all of ! (Can you see why?)

Can you generalize to higher dimensions?