Find the values of k for which the quadratic equation 9x² - kx + k = 0 has equal roots.

We know that if D=0 then the equation has equal and real roots.
D = b² - 4ac = 0
a = 9, b = -k, c = k
Substitute the values we get,
(-k)² - 4 × 9 × k = 0
=> 9k² - 36k = 0
=> 9k(k - 4) = 0
∴ k = 0 or k = 4