Question

The focal length of the convex lens is positive or negative

Answer 1

you can easily verify that the focal length is in the positive x direction for a convex lens while it is along the negative x direction for a concave lens. So f is positive for a convex lens and negative for a concave

Answer 2

Here is your answer.

The focal length is the negative for convex lens.

It is positive for cancave lens.

Hope that this helps you.