Which property of polynomial addition says that the sum of two polynomials is always a polynomial

Astrid spent $36 on new shoes. this was $12 less than twice what she spent on a new skirt. part a: which equation would solve for how much she spent on the skirt? part b: solve the equation in the workspace provided below. how much did astrid spend on her skirt?

Would greatly appreciate these 3 answers

With this question, the answer with the red arrow is incorrect!