\text { Question 6. Let } M=\left[\begin{array}{lll} 1 & 1 & a \\ 1 & b & b \\ c & c & c \end{array}\right] Find all possible combinations of

values of a , b , and c so that M is invertible. [Use row reduction, and your understanding of how a row echelon matrix will establish that M is not invertible.]