Find in the simplest form: \( (x-3)(x+3)-x(x+1) \)

To simplify the expression \( (x-3)(x+3)-x(x+1) \), we start by expanding both parts. First, we expand \( (x-3)(x+3) \): \[ (x-3)(x+3) = x^2 - 9 \] Now, we expand \( x(x+1) \): \[ x(x+1) = x^2 + x \] Now we can substitute these expansions back into the original expression: \[ x^2 - 9 - (x^2 + x) \] Distributing the negative sign gives us: \[ x^2 - 9 - x^2 - x \] Next, we can combine like terms. The \( x^2 \) terms cancel each other out: \[ -x - 9 \] Thus, the simplest form of the expression is: \[ \boxed{-x - 9} \]