1. The question "why we plug in -1" usually arises when solving equations or evaluating functions. 2. Plugging in a value like $-1$ means substituting $x = -1$ into the expression or equation. 3. We do this to check if $-1$ is a solution or to find the value of the function at $x = -1$. 4. For example, if we have $f(x) = x^2 + 3x + 2$, plugging in $-1$ means calculating $f(-1) = (-1)^2 + 3(-1) + 2$. 5. This helps us understand the behavior of the function or verify solutions. 6. In summary, plugging in $-1$ is a method to test or evaluate expressions at that specific value.