A book is placed 25 cm from an eyeglass lens with a power of 2.5d. Where is the image formed

Would like to know how to find it. The answer is given as -67 cm.

The focal length of a lens in meters is the reciprocal of its power in dioptres:

f = 1/(2.5 dioptre) = 0.4 m = 40 cm

then you use the lens equation:

1/o + 1/i = 1/f

You know the object distance (o) and the focal length (f). Solve the equation for the image distance (i):

1/i = 1/f - 1/o
i = 1/(1/f - 1/o)

and plug in your numbers:

i = 1/( 1/(40 cm) - 1/(25 cm) )

i = -67 cm

The negative sign means that it is a virtual image formed in front of the lens