Question

Mohan secured concave mirror suggest a method to find its focal length accurately

Answer 1

if he used a concave mirror then he must pass the light along its principal Axis parallel to the principal axis and if the paraxial rays are exactly parallel to principal Axis then it must converge to its focus because all paraxial rays parallel to principal Axis convert to focus and the distance from home to focus is focal length that's how you can find focal length accurately in a concave mirror