Please help!!
1) First multiply the matrix out and get the conditions a+2b-c = 0. Then try to prove both closure under addition and closure under multiplication.
2)Both fail.
i) $\displaystyle \displaystyle A = \left[ \begin{array}{cc} 1 & 0 \\
0 & 1
\end{array} \right] $ and $\displaystyle B = -A$
ii) $\displaystyle \forall A \in S$,$\displaystyle \lambda = 0 \Rightarrow \lambda A = 0 \notin S$