hiii,

Here is the solution to the question

Distance between object and image = 10 cm

Magnification = height of image / height of object =  v/u

M= 1/4 (given)
m= v/u =1/4
     u = 4v

u-v = 10cm
so, 
4v-v = 10 cm
   v= 10/3 cm
 hence u =40/3 cm

now applying the lens formula

1/f = 1/v -1/u
     = -3/10 + 3/40
     = - 4.4 cm

therefore the focal length is -4.4cm

I hope you understand the solution.
Feel free to revert back for any queries.

 Thank you!!!