Right but he doesn't have a problem with (a,0,0), he already knows and proved it as he stated above, what he wants to know is how is it that vectors of the form (a,b,0) are not a subspace? because the ACTUAL Answer in the TEXTBOOK IS it ISN'T a subspace.
Three Blocks total, Two on sides hanging off the table, and one Block on table in the middle of the two outer blocks. . . symmetrical.
block1 from left has a mass of M, hanging and connected to a pulley, which is the connected to Block 2(on a table) with Mass of 2M, Block three is also...