Work done required to stretch the spring from cm to a length of cm is J.

Consider the natural length of the spring as m.

Work done in moving the object from to  is .

Here , m and m.

From the Hookes law , .

Substitute , , and in .

. 

Work done required to stretch the spring from cm to a length of cm is J.

Here , m and m.

Substitute , , and in .

.

Solve the equations and to get and values.

Substitute in .

.

Therefore the length of the spring is .

Natural length of the spring is .