p1(A) = AA - 9I,so that
p2(A) = A - 3I, and
p3(A) = A + 3I,
p2(A) p3(A) = (A - 3I)(A + 3I) =(here I stands for the (square) identity matrix the same size and shape as the original square matrix A).
= AA - 3(IA) + 3(AI) - 9(II) =
= AA - 3A + 3A - 9I =
= AA - 9I = p1(A)