One of the five quadratics below has a repeated root. (The other four have distinct roots.) What is the repeated root?
-x^2 + 18x + 81
3x^2 - 6x + 9
8x^2 - 30x + 32
25x^2 - 30x - 9
x^2 - 28x + 196
The repeated root is 4.