1. The problem is to solve the system of equations: $$2y = 7x - 9$$ $$2y = 2x - 4$$ 2. Lulu uses substitution by first solving for $y$ from the first equation: $$2y = 7x - 9 \implies y = \frac{7x - 9}{2}$$ 3. She then substitutes $x = -1$ into the first equation to find $y$: $$2y = 7(-1) - 9$$ $$2y = -7 - 9$$ $$2y = -16$$ $$y = -8$$ 4. Next, she sets the two expressions for $2y$ equal to each other to solve for $x$: $$7x - 9 = 2x - 4$$ $$7x - 9 = 2x - 4$$ $$7x - 2x = -4 + 9$$ $$5x = 5$$ $$x = 1$$ 5. Lulu's mistake is in section B where she incorrectly simplifies the equation: She wrote: $$5x - 9 = -4$$ $$5x = -5$$ $$x = -1$$ But the correct simplification is: $$7x - 9 = 2x - 4$$ $$7x - 2x = -4 + 9$$ $$5x = 5$$ $$x = 1$$ 6. Therefore, the correct solution is $x=1$, $y=\frac{7(1)-9}{2} = \frac{7-9}{2} = \frac{-2}{2} = -1$. 7. Lulu's final answer $(-1, -8)$ is incorrect. **Final conclusion:** Her mistake first occurred in section B.