Answers
Part 1: 36z²-9z+8
Part 2: 5z³+4z²+16z-23
Part 3: Yes
Explanation
Part 1
(11z²-4z+2) + (-2z+3+12z²) + (3-3z+13z²) = 11z²+12z²+13z²-4z-2z-3z+2+3+3
= 36z²-9z+8
Part 2
(5z³ + 40z² + 7z − 15) - (36z²-9z+8) = 5z³ +40z²-36z²+7z+9z-15-8
=5z³+4z²+16z-23
Part 3
Yes.
The polynomials 5z³+4z²+16z-23 and 36z²-9z+8 form a system analogous to the integers, which are closed under the operations of addition and subtraction.