1. The problem asks to identify where Emilio made a mistake while factoring the trinomial $x^2 - x - 12$. 2. In Part A, Emilio rewrites the expression as $x^2 - 4x + 3x - 12$ to regroup terms. 3. This regrouping is correct because $-4x + 3x = -x$, so the expression remains equivalent. 4. In Part B, Emilio uses the area model with columns labeled $x$ and $+4$, and rows labeled $x$ and $-3$, filling the cells with $x^2$, $+4x$, $-3x$, and $-12$. 5. However, the terms $+4x$ and $-3x$ sum to $+x$, not $-x$, which contradicts the original trinomial. 6. This indicates the mistake first occurred in Part B, where the signs of the terms were incorrectly assigned. 7. In Part C, the factored form is given as $(x - 3)(x + 4)$, which corresponds to the terms in Part B, so the error propagates here but did not originate here. **Final answer:** Emilio's mistake first occurred in Part B.