a) Prove, by completing the square, that the roots of the equation x2 + 2kx + c = 0, where k and c are constants, are −k ± √(k2 − c).
(4)
The equation x2 + 2kx ± 81 = 0 has equal roots.
(b) Find the possible values of k.
i don't understand how to do part b .