Question

Suppose that A\(\in L(C^3)\)is a self-adjoint operator. Suppose that Tr(A)=4
A(1,0,0) = (1,0,0)
(1,2,0) is an eigenvector of A
a) What is the eigenvalue that corresponds to the eigenvector (1,2,0)?
b) find the matrix of the operator a in the standard basic of \(C^3\)
Please, help