The discriminant d = b^2 - 4ac must be zero: (-4k)^2 - 4*4k = 0 16k^2 = 16k k = 1 k = 0 --- degenerated case. If k = 1, then the equation is x^2 + 4x + 4 = 0, = 0 ====> The solution is x= -2.