I am very confused with the subset tests an i have these three questions that i cannot do

can someone help me out please.

i need to show if w is a subspace of v or not.

1) V

= M2,2(R) and W = {A 2 V | AT = A}.

2) V = R3 and W = {(x1, x2, x3) 2 V | 3x1 + x2 − x3 = 1}.

3) V = P2(R), real polynomial functions of degree at most 2, and W= {f 2 V| f(−2) = 0}.

or send me a link from wher ei can get a better understanding of subset. cheers