To add the polynomial expressions \( 5z - 2z^2 - 4z + 5 \) and \( 6z^2 + 3 - 7z \), we first combine like terms from both expressions.
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Combine like terms from the first expression: \[ 5z - 4z - 2z^2 + 5 = (5z - 4z) + (-2z^2) + 5 = 1z - 2z^2 + 5 = -2z^2 + z + 5 \]
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Now write the second expression: \[ 6z^2 - 7z + 3 \]
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Now add both expressions: \[ (-2z^2 + z + 5) + (6z^2 - 7z + 3) \]
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Combine like terms:
- For \( z^2 \) terms: \[ -2z^2 + 6z^2 = 4z^2 \]
- For \( z \) terms: \[ z - 7z = -6z \]
- For the constant terms: \[ 5 + 3 = 8 \]
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Putting it all together, we have: \[ 4z^2 - 6z + 8 \]
Therefore, the sum of the polynomial expressions is: \[ \boxed{4z^2 - 6z + 8} \]