We write the expressions in separate rows, keeping like terms in the same column:

        9xy -3yz + 2zx
    +  9xy        + 2zx - 4y
     + 3xy         - 2 zx + 5x
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   21xy -3yz + 2zx -4y +5x
In the above example, -4y and 5x are carried over as they are, because they have no like terms. II. xz is same as zx.

We know that subtraction of a number is the same as addition of its additive inverse. We have
8x² -5xy + 9y²+6x - 4y
6x²          - 5y²       + 7y - 4
(-)          (+)      (-)     (+)
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 2x² -5xy + 14y²+6x -11y + 4

Coefficient of x²y² is 1.

Coefficient of x²y is –10. Coefficient of xy² is 5.