Joey did not complete the problem correctly. His error occurred during the simplification step where he combined the terms.
Here's what he wrote:
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He started with the correct expression: \( (2x)(x) + (2x)(12) + (-5)(x) + (-5)(12) \), which expands and gives: \[ 2x^2 + 24x - 5x - 60 \]
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Next, when combining the like terms \( 24x \) and \( -5x \), Joey should have calculated: \[ 24x - 5x = 19x \] However, he mistakenly wrote \( 21x \) instead of \( 19x \).
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Thus, Joey’s final expression of \( 21x - 60 \) is incorrect.
Erin completed the problem correctly. She properly tracked the initial terms and correctly combined \( 24x - 5x \) to get \( 19x \). Her final expression \( 2x^2 + 19x - 60 \) is the correct product of the binomials.
In summary, Joey's mistake was in the addition of the coefficients of the \( x \) terms, where he wrote \( 21x \) instead of the correct \( 19x \).