Answer:
f = 1.255 cm
Step-by-step explanation:
The Radius of Curvature:
The radius of that hollow sphere, whose part is the spherical mirror, is known as ‘The Radius of Curvature’ of mirror.
Focal Length:
The distance between principal focus and pole is called ‘Focal Length’. It is denoted by ‘F’
The focal length is basically equal to the half of the radius of curvature of the mirror:

where,
f = focal length = ?
r = radius of curvature = 2.51 cm

f = 1.255 cm