I am not able to compute the REF or the RREF, the a confuses me with the calculations. I do know how to check for consistency and inconsistency but I couldn't perform any row operations due to the presence of a
$$\begin{cases} 2_1 + _2 − _3 = 0\\ _1 + 2_2 + _3 = 0\\ −2_2 − 2_3 = 0\\ a_1 + _2 + _3 = 0 \end{cases}$$
a) For which values of the scalar is this system consistent and exactly determined?
b) Solve the system, using the value for that makes it consistent and undetermined.