Using the formula for the combination of lenses: 1/F = 1/f₁ + 1/f₂. A convex lens has a positive focal length (+10 cm), and a concave lens has a negative focal length (-6 cm). 1/F = 1/10 + 1/(-6) = (3 - 5)/30 = -2/30 = -1/15. Therefore, F = -15 cm.