For the first question I sent from my Physics practical

Place the convex lens on a plane mirror, which is resting on a horizontal surface.

Hold an object pin vertically above the convex lens. Adjust its height until its tip appears to coincide with the tip of its inverted image, as seen by looking vertically down through the lens.

Move your head slightly from side to side. If there is no relative movement between the object pin and its image, then the condition of no parallax has been achieved. If there is parallax, adjust the height of the object pin until no parallax is observed.

Once no parallax is achieved, measure the vertical distance from the tip of the object pin to the optical center of the convex lens using a meter rule. This distance is the focal length ($f$) of the convex lens.

The focal length of the convex lens is the measured distance from the object pin to the optical center of the lens when no parallax is observed.