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Since the square roots of 196 are +14 and -14,
x2 + kx + 196 = 0
to be a perfect square trinomial, it would have to factor as
(x+14)2 = 0 or (x-14)2 = 0
or
(x+14)(x+14) = 0 or (x-14)(x-14) = 0
which when multiplied out gives
x2 + 28x + 196 = 0 or x2 - 28x + 196 = 0
So k would have to be +28 or -28
Edwin