Matrices And Determinants

Let $$A=\left[\begin{array}{lll}2 & a & 0 \\ 1 & 3 & 1 \\ 0 & 5 & b\end{array}\right]$$. If \(A^3=4 A^2-A-21 I\), where \(I\) is the identity matrix of order \(3 \times 3\), then \(2 a+3 b\) is equal to