Can you help with some algebra?

1)What must be added to 5x^3 - 3x^3 + 7x - 8 to give 3x^3 + 2x^2 - 8x + 15?

2) Given that A+ 3x + 5y + z and B = 8x - 3y - 5z, find: i A+B ii A-B

3) From the square of -5x, take away the square of -3x

4) Explain why each statement is incorrect, and write the correct answers:

a. 4 X 2k + 3k = 20k

b. 40mn / 5m X 2n = 4

c. 8 X 4p -5p X 3 = -24p

d.8z + 18z^2 X 6/ 3z = 9z

6) a. simplify 5a X 7b

b. simplify 7b X 5a

c. Is 5a X 7b = 7b X 5a

d. does it matter in which order the terms are divided?

7) a. simplify 28a / 7

b. simplify 7/ 28a

c. is 28a /7 = 7/ 28a

d. does it matter in which order the terms are divided?

Sorry for all the questions but please explain as well as you can... Some of them I have done already but just want to check that the working and stuff is all good! THankyou for any and all help!! xoxoxo