Derivative of y=(x+5)(x-4)

1. Apply the product rule:

   $$\frac{d}{d x} f{\left(x \right)} g{\left(x \right)} = f{\left(x \right)} \frac{d}{d x} g{\left(x \right)} + g{\left(x \right)} \frac{d}{d x} f{\left(x \right)}$$

   $f{\left(x \right)} = x + 5$; to find $\frac{d}{d x} f{\left(x \right)}$:

   1. Differentiate $x + 5$ term by term:

      1. Apply the power rule: $x$ goes to $1$

      2. The derivative of the constant $5$ is zero.

      The result is: $1$

   $g{\left(x \right)} = x - 4$; to find $\frac{d}{d x} g{\left(x \right)}$:

   1. Differentiate $x - 4$ term by term:

      1. Apply the power rule: $x$ goes to $1$

      2. The derivative of the constant $-4$ is zero.

      The result is: $1$

   The result is: $2 x + 1$

The answer is: