(3-8z)(1-2z)
Factors of 3 are 3 and 1, so we place 3 in one parenthesis and 1 in the other.
Factors of 16 are 16 and 1, 4 and 4, and 2 and 8, and the sign of z² is + and the middle term is negative, which tells us that we are looking for (3-f₁z)(1-f₂z) where f₁ and f₂ are factors of 16 such that 3f₂+f₁=14. The only values of f₁ and f₂ that fit are f₁=8 and f₂=2.