The length of a steel wire when stretched by a force F1 is l1 and when stretched by a force F2 is l2.The unstretched length of the wire is
l1F2−l2F1F2−F1
l2F2−l1F1F2−F1
F1(l2−l1)F2−F1
F2(l2−l1)F2−F1
Δl=FlYA⇒Δl∝F
Therefore,
l1−lo=F1 and l2−lo=F2
⇒lo=l1F2−l2F1F2−F1