Please check.
(-6m^2n+3mn^2-5mn+2)+(4m^2n+2mn^2-6mn-9)
Answer: -2m^2n+5mn^2-11mn-7
(2x^4-3x^2+7x)-(5x^3+2x^2-3x+5)
Answer: 2x^4-5x^3-5x^2+10x-5
(a-8)(a-1)
Answer: a^2-9a+8
(z+6)^2
Answer: z^2+12z+36
(5x+2y)^2
Answer: 25x^2+20xy+4y^2
Is that order okay? Does it matter?
(2x-7)(2x+7)
Answer: 4x^2-49
tstsftfjfstafjscfqtf
Yes, the order of terms in the final expression does not matter. So, whether you write it as -2m^2n+5mn^2-11mn-7 or -11mn-2m^2n+5mn^2-7, both are correct.
In the second expression, your answer is correct: 2x^4-5x^3-5x^2+10x-5.
For the third expression, your answer is also correct: a^2-9a+8.
In the fourth expression, your answer is correct: z^2+12z+36.
For the fifth expression, your answer is correct: 25x^2+20xy+4y^2. And no, the order doesn't matter in this case either.
In the last expression, your answer is correct: 4x^2-49.