To complete the square for the polynomial \(t^2 + 12t\), we need to find a number to add that will make it a perfect-square quadratic.
- Start with the coefficient of \(t\), which is 12.
- Take half of this coefficient, which is \( \frac{12}{2} = 6 \).
- Square this result: \(6^2 = 36\).
Now, we can express the original polynomial:
\[ t^2 + 12t + 36 \]
This can then be factored as:
\[ (t + 6)^2 \]
Thus, the number you need to add to make the polynomial a perfect-square quadratic is \(36\).