Given -1 and 1 are two eigen values of matrix A
and 18 is the eigen value of A^2 + 3A
so other eigen values of A^2 + 3A are 4 , -2
and other eigen value of A is 3 ( 3^2 + 3 )
Eigen value of A is -1 ,1, 3 and A^2 + 3A is 4,-2,18
No eigen value is 0 so both are invertible