Let focal length of converging and diverging lenses be + f and - f respectively.
Power of converging len P1 = 1/f, Power of diverging lens P2 = - 1/f
∴ Power of combination P = P1 + P2 = 1/f - 1/f = 0
∴ Focal length of combination F 1/P = 1/0 = ∞ (infinite)
F = ∞.