One root of $x^2 - 9x + k = 0$ is twice the other. What is the value of $k$?

Explanation

1. 01

   Let the roots be $r$ and $2r$. Their sum is $3r = 9$, so $r = 3$.

2. 02

   The roots are 3 and 6, and $k$ is their product: $k = 18$.

3. 03

   Check: $x^2 - 9x + 18 = (x - 3)(x - 6)$.