To complete the square for the quadratic polynomial \( k^2 - 30k \), we follow these steps:
- Take the coefficient of \( k \), which is \(-30\).
- Divide it by 2: \(-30 / 2 = -15\).
- Square the result: \((-15)^2 = 225\).
Now, we fill in the blank:
\[ k^2 - 30k + 225 \]
Therefore, the number that makes the polynomial a perfect-square quadratic is \( 225 \).
The completed square form of the polynomial would then be:
\[ (k - 15)^2 \]