Answer:
See below ~
Step-by-step explanation:
Part A :
⇒ Total Length = Side 1 + Side 2
⇒ Total Length = (8x² - 5x - 2) + (7x - x² + 3)
⇒ Total Length = 7x² + 2x + 1
=============================================================
Part B :
⇒ Length of 3rd side = Perimeter - Part A solution
⇒ Length of 3rd side = 4x³ - 3x² + 2x - 6 - (7x² + 2x + 1)
⇒ Length of 3rd side = 4x³ - 10x² - 7
===========================================================
Part C :
⇒ Yes
⇒ It shows they are closed under addition and subtraction because no new values come into the equations
