For which values of $k$ does $9x^2 + kx + 4 = 0$ have equal roots?

Explanation

1. 01

   Equal roots: $k^2 - 4 \times 9 \times 4 = 0$, so $k^2 = 144$.

2. 02

   Hence $k = 12$ or $k = -12$.

3. 03

   Check: $9x^2 \pm 12x + 4 = (3x \pm 2)^2$.