For (a), I get $a = -c + d, b = (2/3)c$;  $c$ and $d$ are free variables.   Not sure if I’m right.
For (b), I’m not sure what’s it asking for $C$. Can I just say $C$ is everything other than $B$?
Please read the this problem and its solution. This is almost the same problem except the matrix $A$ is different.