To complete the square for the expression \( m^2 - 5m \), we can follow these steps:
- Take the coefficient of \( m \), which is \(-5\).
- Divide it by 2: \(-5 / 2 = -\frac{5}{2}\).
- Square the result: \((- \frac{5}{2})^2 = \frac{25}{4}\).
Thus, the number that makes the polynomial a perfect-square quadratic is \(\frac{25}{4}\).
Therefore, the completed square form of the polynomial is:
\[ m^2 - 5m + \frac{25}{4} = \left(m - \frac{5}{2}\right)^2. \]