Math Is Fun Forum

Hi everyone,
   I'm hoping to get some step by step answers.
An example of my problem is
S={(x+2y,x,-y)^τ  :x,y (I don't see the character to put in here) R}

Now, I have read and reread my text, but I'm just not grasping the steps to figure out if it is a subspace of R^3.   Any suggestions, not necessarily answers, just how to, if possible. Thank you in advance.