we know for triangular/diagonal the determeinant is product of principal diagonal elements so in A-(lambda)I every value of lambda as any of of the principal diagonal elements will give result as 0. so all the diagonal principal diagonal elements are eigen values.

Eigen value $lambda$=1,-1,2,-2 A-$lambda$I=0 just solve it and for the 2nd question i think building a truth table is a naive way of answering it if any1 has better solution then plz reply

if the matrix is either upper triangular or lower triangular matrix then the principal diagonal element are called eigen value.
1,2,-1,-2 for this question ...