a) $A e_n$ picks out the $nth$ column of $A$. Given that you should be able to get $A$ by inspection

b) the quickest way to see if a 3x3 transformation is 1 to 1 is to see if it's determinant is non-zero

c) is trivial

d) row reduce the result from (c) to get the basis of the span of $T_A \circ T_B$