The determinant of the matrix is calculated by the characteristic equation, which is derived by finding the eigenvalues of the matrix. Given that the eigenvalues of the matrix are 3 and 6, we can substitute them into the characteristic equation and solve for \( \mu \). After solving, we find:
\[
\mu = 5.
\]
Thus, the value of \( \mu \) is \( 5 \).