M(m-4)+(3-m)(3+m)

To simplify the expression \( m(m-4) + (3-m)(3+m) \), we start by expanding both parts of the expression.

1. Expand \( m(m-4) \): \[ m(m-4) = m^2 - 4m \]

2. Expand \( (3-m)(3+m) \): This is a difference of squares \( (a-b)(a+b) = a^2 - b^2 \) where \( a = 3 \) and \( b = m \): \[ (3-m)(3+m) = 3^2 - m^2 = 9 - m^2 \]

Now combine the results: \[ m^2 - 4m + 9 - m^2 \]

The \( m^2 \) terms cancel each other out: \[ -4m + 9 \]

Thus, the simplified expression is: \[ \boxed{-4m + 9} \]