Write a system of equations that has the given augmented matrix.

a. \(\left[\begin{array}{lll:l}6 & 7 & 8 & 3 \\ 1 & 2 & 3 & 4 \\ 0 & 1 & 3 & 4\end{array}\right] \)

b. \(\left[\begin{array}{lll:l}1 & 0 & 0 & 3 \\ 0 & 1 & 2 & 4\end{array}\right] \)

c. \(\left[\begin{array}{rrr:r}1 & 0 & 0 & 32 \\ 0 & 1 & 0 & 27 \\ 0 & 0 & 1 & -5 \\ 0 & 0 & 0 & 3\end{array}\right]\)