6. 6. Fill in the five entries: a,b,c,d,e in the following 4x4 matrix

\[ \begin{bmatrix} 1 & -1 & a & 5 \\ b & 4 & c & 8 \\ 2 & -7 & -1 & d \\ e & -1 & 6 & 3 \end{bmatrix} \]

so that the matrix is symmetric.(15分)