Please help and explain <3 ~

Use the x-intercept method to find all real solutions of the equation. -9x^3-7x^2-96x+36=3x^3+x^2-3x+8

Marla bought a book for $12.95, a binder for $3.49, and a backpack for $44.99. the sales tax rate is 6%. find the amount of tax and the total she paid for these items

How can you tell when x and y are not directly proportional?